Taking a P switch and jumping out, throwing it into the air and landing on it on your decent to activate it and make a bridge bellow you.
Button Input Header
Grabbing the P switch with Y, jump off the ledge going right (or left depending on level) and at the peak of your jump throw the P switch into the air. Keep holding down the B button and as you descend, Mario will land on the P switch as he is falling. Once you land on the P switch, hold down Y to make it over the bridge that was created from the P switch. If you are repress Y before you land on the P switch you will instead grab it and no land on it.