Get to the part where you have to find Major Raikov and take his uniform. Dress up like Ivan Raikov and go to the other gaurds. They must salute to Ivan. During that time, go into first person view and aim at a guard's crotch and punch them. The guard will grab his crotch and fall face down, knocked out. This can also be done when you go into the research facility where you must find Granin. Dress up like a scientist and punch the guards or other scientists in the crotch.