Ragdoll physics is a new feature implemented in Grand Theft Auto IV. With ragdoll physics, players and NPCs lie on the ground dynamically. The affected model essentially behaves like a "ragdoll", hence the name. Characters no longer lie flat and static on surfaces. Their bodies will conform to any objects realistically and can be moved around by collisions and bullets.
Ragdoll physics are used extensively in GTA4, where a player may become a ragdoll by falling from an excessive height (approximately two or three stories), tripping or mis-stepping (occurs especially when drunk), being knocked out or killed, or being hit by an object, melee weapon (includes unarmed attacks), vehicle, or ragdoll. However, the player does not always become limp when a ragdoll (see Involuntary Ejection), often playing an animation which may be broken by death or affected by physics. For example, Niko Bellic tends to curl up in an attempt to protect himself when first jumping from (or being ejected from) a vehicle. This is most obvious when jumping from a helicopter at a high height, as Niko is never seen limp in mid-air unless he is dead or unconscious.