The shape your pony makes over a fence is called a bascule. A good bascule is a rounded arc – similar to the shape a dolphin makes as it jumps out of the water.
A poor bascule is when your pony jumps flat, often with his head in the air and a hollow back. This means he can’t jump as high, so he’s more likely to knock down poles. Improving your pony’s bascule produces a better jump, which helps leave the fences up and allows you to move up the levels!