A bail out refers to an act of jumping from a moving vehicle. This ability was first made available in Grand Theft Auto: Vice City, and has been available in most Grand Theft Auto games since. Until GTA Vice City, the player could only exit vehicles once they were traveling at low speed or had come to a complete stop (with the exception of ships).
Bail-outs are often used as a quick means of escaping from a vehicle which is either in danger of exploding, entering dangerous territory, or about to fall off a cliff or into water. Bailing out can also be strategically useful, allowing one to send a vehicle speeding into a group of enemies while leaving oneself free to attack independently. Bail-outs are executed with the same key or button that the player uses to enter or exit vehicles, while the vehicle is still in motion. In most cases, the player will lose a small amount of health as a result of hitting the ground after exiting the vehicle. Grand Theft Auto: San Andreas is an exception — the player will not lose health when bailing out unless they are bailing out of a land vehicle which is in mid-air (and which is relatively high above the ground).
In GTA IV, the use of ragdoll physics makes it hard to predict the amount of damage the player will sustain, as the player will now maintain inertia from the vehicle and continue to move forward, bouncing along the ground and losing health with each bounce; the damage sustained is generally greater than that suffered in previous GTA games, to the point of being potentially lethal to players lacking in health. As inertia will still move the player forward after bailing out, and bail-outs are executed at least one second after the player presses the button, players must take care to bail out well before a vehicle is due to explode or fall.
Bail-outs can be done from aircraft as well as land vehicles. As falls from high altitudes are lethal (unless the player lands in a body of water), players who completed the Flying School in GTA San Andreas are equipped with parachutes after exiting any aircraft, to provide a chance of survival after ejecting. An exception to this could be cars and trucks under the influence of the "Flying Vehicles" cheat- It is impossible to exit one if it is moving, even if it is submerged in water.