In order to see this glitch, play through the level until you see a Flood Combat Form, then approach until it starts a melee attack. As the arm swings back, shoot off the arm whip it would use. Although the arm whip is gone, you can still be damaged as if it were still there. This is a difficult trick to pull off, as the attack is usually quick, but with quick timing and good aim, you will be able to do it.
In Halo 3, lure a Combat Form to your position and get it to charge at you then shoot off it's whiparm and allow it to get close to you. It will melee you and you will take damage even if the whip is gone. Then the Flood Combat Form will just follow you around. Another form of this glitch is when you melee the whip off a flood combat form wielding a weapon if you have a brute Spiker shoot at the whips old position and the spikes will levitate in mid air