From Halopedia, the Halo wiki
Some uses of switches include doors, elevators, vending machines, and even deathtraps. Switches typically fall into three categories: flow switches, which aid or impede a player's movement from one location to another; damage switches, which cause harm to a player; and supply switches, which aid a player, typically by dispensing some item or weapon.
Switches may also be categorized by the techniques used to build them. The most common techniques are min/max, wire, magnetism, and roller.
Min/max switches (also called Instance Switches) earn their name from Forge objects' Run-Time Minimum and Run-Time Maximum settings; these are the settings that allow the switch to work. A min/max switch is basically a mechanism that relies on the destruction of an object; when the object is destroyed, the Run-Time Minimum setting forces a different instance of the object to spawn elsewhere. The most commonly-used objects in min/max switches are Fusion Coils, Grav Lifts, and Pallets, because they are all easily broken/blown up.
A simple example can be constructed using Grav Lifts. Start by placing two Grav Lifts. Change the first Grav Lift's settings so that its Place at Start is set to Yes and its Respawn Time is set to Never. Edit the second Grav Lift so that its Place at Start is set to No and its Respawn Time is set to 180.
Then, change the Run-Time Minimum for Grav Lifts to 1, and the Run-Time Maximum to 2. Start a new round, and shoot the first Grav Lift until it breaks. That Grav Lift's destruction will force the other Grav Lift to spawn.
The run-time minimum is a number that when the amount of that specific object falls below it, the object(or at least one instance of it) respawns. In short, the number of objects will not go below the run-time minimum. The run-time maximum, on the other hand, is how much of that object can be placed on the map. An object will not respawn or spawn if the object count is already equivalent to the run-time maximum. Every other term is self-explanatory.
This is how it works: one Grav Lift is placed at the start, and it will never respawn. When you, or any other unseen force of nature and physics, break the gravity lift, the object count falls below the run-time minimum, and the game knows that it should spawn another. It knows it cannot spawn the first grav lift, so it spawns the second.
Wire switches, also known as touch switches, earn their name from how they work.
In Halo 3, when an object is undisturbed for several seconds, the game's physics engine and collision detection system ignore it. Such ignored objects are not affected by Man Cannons, or gravity, though gravity lifts will still have an effect. If, however, these objects are touched or hit by some force (be it a player, a bullet, an explosion, or anything else), then they stop being ignored, and Grav Lifts, Man Cannons, and gravity once again begin to affect them.
Wire switches take advantage of this phenomenon. The basic setup is to have a movable object spawn and remain undisturbed for long enough that it becomes ignored. After that time, a Grav Lift or a Man Cannon should spawn near the object; because the object is ignored, the lift or cannon will not affect it. To activate the switch, the player must simply disturb the object -- move it, shoot it, or have it touch something.
A simple example can be constructed using Pallets, a Dumpster, and a Man Cannon.
Place a row of Pallets on the ground, each touching end-to-end. At the end of the line, place a Dumpster, which should also touch the Pallets. Place a Man Cannon directly behind the Dumpster; the Dumpster should launch forward. Modify the Man Cannon so that its Place at Start is set to No and its Respawn Time is set to 10. Start a new round.
Wait for the Man Cannon to spawn. When it does, notice how the Dumpster is unaffected by the Man Cannon. This is because the Dumpster, after being undisturbed for several seconds, is now being ignored by the game. Bump the Pallet on the opposite end of the Dumpster. That Pallet should move the one it's touching, which should move the one it's touching, which should... until eventually, the last Pallet bumps into the Dumpster, causing the game to stop ignoring the Dumpster and causing the Man Cannon to launch the Dumpster into the air.
Magnetism switches are quite simple to create and use. They take advantage of a useful trait of Grav Lifts: a Grav Lift, in addition to lifting objects above it, will also affect objects immediately below it. Man Cannons have the same property, but the pull below a Man Cannon's base is less intense.
Float a Bridge (the immovable Forge object) in the air, and place a Grav Lift on top of the Bridge. Return to Player Mode, and walk under the bridge. Jump up against the bottom of the floating Bridge, where the Grav Lift was placed. You should be held against the bottom of the Bridge, as the Grav Lift above tries to continuously pull you upward.
Roller switches use cylindrical or spherical objects such as Barrels or Propane Tanks to move heavy objects that would otherwise be immovable to a player, such as Barriers or Dumpsters.
Building a switch that uses Roller Technique is very simple. Place several consecutive "rollers" on the ground with a heavy object on top of them, this way the weight of the object is distributed among the rollers and is easier to move.
Resistance technique is one of the most simple and most used switch technique. Resistance switches consist of a movable object being restrained from moving the direction it would naturally go. The object holding back the movable piece (the restrainer) must be strong enough to restrain it while still be able to be removed when necessary.
For this reason, most resistance switches use either the three types of Power-ups or a turret. Power-ups are solid objects but can be picked up, allowing the movable object being restrained to move. There are several forces that can affect the natural movement of an object; Grav Lifts, Man Cannons, and of course gravity itself.
Place a Dumpster with a Custom Power-up on one end of it and a Man Cannon on the other end. When a player picks up the Power-up, the Dumpster will be free to move and will be pushed forward by the Man Cannon.
Key card technique
Key card technique is the use of a power drain (radar jammer works too, but takes a while) thrown/inserted into the switch to cause fusion coils or propane tanks to explode. Pieces of equipment like the power drain remove themselves from play with a small explosion after their effect is done. This small explosion is enough to set off fusion coils and other explosive objects causing a larger explosion which may be used for activating a min/max switch among other things.
Create a room with a few fusion coils in it and a gravity lift that will force spawn another when it breaks. Then throw your power drain into the room through a slot you've made. Then when the power drain explodes shortly after it'll explode the fusion coils and the first gravity lift spawning the second one which lifts the door.
Flip (RB) technique
Flip technique is the use of the Xbox controller's right bumper (RB) to flip an offset vehicle. The movement of this vehicle flipping is usually used to activate the interworkings of the switch. There are many uses of Flip switches. For example, the use of the vehicle switch may be to bump a fusion coil and set off a min/max switch OR it may bump into another movable object and set off the wire for a touch switch.
Place an offset vehicle against a wall. Then place a fusion coil where the vehicle will flip, that way it will be knocked over by the vehicle once flipped. Finally, place a man cannon just past the fusion coil. When a player flips the vehicle over with RB, the fusion coil will be pushed into the man cannon, thus activating the switch.