Also known as a "whiskey compass", these are fairly standard backup instrumentation featured in the vast majority of aircraft of all kinds
this kind of compass consists of a magnet-loaded ball, weighed down at the bottom to keep upright, floating in liquid (usually Kerosene). Around this ball's "Equator" a series of numbers are painted to read out the magnetic heading of the aircraft.
as the vehicle turns, the free-floating ball remains generally fixed relative to this rotation, given how magnets will pull towards polar alignment with a magnetic field (in this case, the Earth's - mostly)
as that rotating object remains relatively fixed within it's casing, the pilot would observe the visible series of numbers and markings on the ball shift across in the OPPOSITE direction relative to his turn
i.e. the numbers would appear to scroll "into the turn" - not "outwards" from it - this is contrary to objects observed outside relative to the windshield, due to the ball being viewed from "behind"
this is not the behavior depicted in ArmA (and Take On: Helicopters also) - in the modelled cockpit, the compass ball turns at 2x the yaw rate of the aircraft, towards the SAME direction the helicopter is turning
this eccentric behavior is then made even more bizarre by how the texture of the ball is made to show the heading numbers arranged "backwards" - effectively (yet mistakenly) correcting for the aberrant motion as to provide valid heading information, albeit "the other way around"
a realistic "whiskey ball" should have the markings painted as if seen in a mirror, with North and South unchanged, but all points in between effectively flipped right-to-left (except unlike in an actual mirror, the numbers themselves are not inverted, just their ordering) - when seen from the "rear" for it sits in front of the pilot, these numbers correspond to the correct heading through the correct way of motion