Proning

From wiki.soldat.nl

Proning is the act of setting one's soldat flat on his chest by pressing, by default, the "X" button. Pressing "X" again or pointing the cursor behind your soldat will have him stand up.

Proning serves various functions. The most common uses are: to avoid bullets (as it narrows your vertical collision area), to increase accuracy (unconfirmed if this is merely a psychological effect or not), to hide (useful in Realistic Mode), and using the Barret M82A1's scope. The prone function can also be used to execute a superman maneuver (proning and jetting in midair), which can pick up a lot of downward horizontal speed.

On top of that, proning on icy polygons, for some deranged reason Michal Marcinkowski apparently had in coding maps, allows soldaten to slide freakishly fast.