From Halopedia, the Halo wiki
If you move to the point where the player model comes in contact with Vergil's, there is a chance that his tentacle will become stuck partially inside his body. This results in a violently thrashing appendage that will launch anyone who gets close enough.
To induce this glitch, you must be on any level with Vergil moving around with the player. Use a car or other object to jump onto the Huragok. This should push him down to the ground because you are simply too heavy for his gaseous sacks to hold up. While he is sliding around on the ground, you should notice that his lowest, and largest, tentacle is in the ground. There is a chance that this will result in the tentacle "glitching" into Vergil. This usually lasts for about 20 seconds.
By making contact with Vergil while his tentacle is whipping around, you may be launched a surprising distance. This is not particularly useful in reaching any secret places because you are launched horizontally. Beware, this push from the Engineer will also do damage to your stamina, perhaps even killing you, but this is unlikely due to the overshield that his presence brings.