When you stay behind some corner (situation), you can fall on left or right side and shoot your enemy, this animations interpolated to lie on side anim as in SMK
http://www.youtube.com/watch?feature=player_detailpage&v=vklOq6KFQy4#t=290s
Good move if you want to suddenly attack the enemy and do not get shot in the head because you will lie on the floor and it adds a surprise effect and dynamics in such situations, when you engage enemy behind corner
Can be assigned on CTRL+SHIF+A/D