When you jump and your jump is interrupted (by block height, for example) you can immediately jump again. Doing so while sprinting in a 2 block tall place can make you reach really fast speeds at the cost of a ton of your hunger and saturation

Edit: just adding a note that on the java version of the game you can gain a bit of speed from sprinting and jumping in a 2 block tall space, but the jumping isn't as frequent and has a short cooldown, while on Windows 10 edition you're able to jump again as soon as you touch the ground.