Sprinting

Sprinting is an upcoming feature in Minecraft Beta 1.8, which allows the player to run two times faster than walking. The player sprints by double tapping W (default) and then holding it. It will be possible to jump up to four blocks (instead of the usual one block) by sprinting.

Sprinting will be unlimited, but will decrease your hunger bar slowly. When a player sprints, particles will come from the players feet. When a player hits any passive animal (excluding squids), the animal will start sprinting away from the player.

If you hit a mob while sprinting, the mob will fly back.

Sprinting seems to stop when you jump. Sprinting stops when you run into a block or parry with a sword. You do not stop sprinting when you run into a mob.