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. However, unlike other games that use ragdoll physics, characters in GTA IV do not seem to be affected to the fullest extent and still conform to some preset animations, thus assuming a more "human" posture and not like a dummy being thrown around (eg. when you jump off a tall building, the subject would land straight then topple over, rather than landing on one foot and immediately falling to the ground).
When the ragdoll affects on NPC or antagonist is activated by having them hit by a car, falling, etc., they will not just flap around like a "lifeless" ragdoll if they are still alive; they will brace themselves while getting thrown about. When an antagonist or NPC gets hit by with a car (but doesn't die from the impact), they will brace themselves as they are getting thrown about the road. Up until they are killed, they will continue to brace their face with their arms and their legs in the fetal position. If an antagonist or NPC is killed by getting hit by a car, their body will be thrown about lifelessly without bracing. This is added for realism as real life people will automatically brace themselves whenever they are thrown. Ragdoll physics are a bit inconvenient at certain times, especially when bailing out of a vehicle or ejecting involuntarily out of a vehicle, unlike the previous renditions, as the player may continue to flop around until death. This feature adds realism and a sense of more control in the game.
Physics in older games
As ragdoll physics were added in Grand Theft Auto IV in previous games if a player or NPC falls of a building or hill they would fall completely flat even if the ground below is steep. Also if a player or NPC falls on the ground he will land to feet on the ground and then fall flat, even if the player or NPC dies on the impact of the fall they will still hit the ground the same way. In Grand Theft Auto IV if the player falls on a low jump Niko will do a stunt roll but in previous games the player will fall either flat on their feet able to keep moving with a slight bend forward (this is very hard to notice but still there) in Grand Theft Auto San Andreas. If a player or NPC is hit by a car in a game previous to Grand Theft Auto IV they will fall on their back and then get back up if not killed or they will fall the same way but slide over the top of the car, in Grand Theft Auto IV Niko or an NPC will fly over the top of the car and can hit the ground in many different ways depending on how high they fall the speed of the car and the angle of the collison.