The slopes are designed in such a way that you are projected into the air. If the ramp is curved, then there is a higher chance that you will be projected directly above the slope. And this is where our second example comes into play. If the ramp is a half-pipe, then you go up and then you come down. And as you come down, you hold the momentum that you initially had which you can use to go up the second ramp and do this process back and forth.