When crouched and with your rifle up, rapidly aiming left and right will move you in different directions depending on how you do it, which if you are looking left and right rapidly will result in sporadic movement. If done at more realistic reaction speeds it will likely only shift you slightly, but if purposefully done it looks just strange.

This can only occur with your rifle up, and in the normal crouch position, tested with gun down and in other positions, and it doesn't result in any unexpected behavior. It can be done while aiming down the sights or not, with the exact same results. Best replication results are looking back and forth about 90 degrees at a pace of more than 1 back and forths a second. Movement seems less likely if done in a circular motion but does still occur. For easier replication recommend a friend to observe you, pressing yourself up against a wall, or aligning two objects on your screen, then attempting to replicate.

I have heard reports this may even cause you to warp through walls, and though I couldn't replicate that behavior; I did manage to injure myself doing so against certain walls. Doing it too rapidly may only result in you spinning relatively in place, as will only turning in a single direction.