From the Bug Tracker record:

Playing with glovepie, I've found that the lean of a soldier is not checked for big values. If you put 'faketrackir.x=1', the soldier will lean roughly 90 degrees. And can fire from that position. Bigger values will let the upper body rotate more and going in a bigger spiral far from the center. With a value of 30, you'll get a soldier 50cm higher than normal. In MP, other players will not see you deformed in such way, just lean as normal, but you can fire from the point you see locally. It's a cheat in DM/CTF because you can hide behind a corner and lean more than the others can see.

The images show an SLA soldier in the same MP session as the US one. The former is looking left an house corner, the latter on the right, but with his TrackIR X axis set to 38 with GlovePIE. The US soldier can kill the SLA one without being seen.