From Halopedia, the Halo wiki
In order for the glitch to occur one needs merely multiple pod infectors to latch onto Rtas 'Vadum while his energy shields are down. This will cause Rtas to fly upwards and seemingly float. It is most easily seen during The Oracle where only pod infectors and no other enemies are encountered in decent numbers though will also happen easily at the end of Sacred Icon as well. While the glitch works best with Rtas due to his invincibility, it works with most other AI as well, although they usually die before they attain much altitude. It will not work on Brutes and the Flood.
Start at the beginning of The Oracle and reach the area where [Sesa 'Refumee|Heretic Leader]]'s hologram appears. After Rtas 'Vadum finishes talking with the hologram, Pod infectors will come from the upper levels behind the Covenant unit. Whenever a Pod infector jumps onto Rtas, he will attempt to kill it using his Energy Sword or whatever other weapon he is holding. After enough Pod infectors attack him, and after he loses his shields, he will fly through the air, seemingly floating, a bizarre response to being attacked by popping Pod infectors. You can speed up the process by attacking Rtas and lowering his shield themselves, allowing the pod infectors to latch on faster.
While not as fast, another way to see the glitch is to play through Sacred Icon and reach the final battle at the camp. During the course of the battle Flood carrier forms will spawn and approach you or your allies including Rtas. While one can lower Rtas's shields and force the carriers to explode, if Rtas holds an energy sword he will often charge at them himself allowing both the carriers to lower his shields with their explosions and the pod infectors to latch on afterwards.