First you have to find a pipebomb (or propane tanks), then you get on a high building and look as far down as you can.

Face your back in the direction you want to go. Throw the pipe bomb staright down and then press Y as fast as you can and don't stop.

When the bomb explodes your character will start stumbling backwards (keeping pressing Y or he will stop and fall).

Once in the air you will have to stop pressing Y and shoot, and then go back to pressing Y or else the character will take a break and fall.

This cheat is helpful on the Sugarcane Field. If you position him right you can land on top of the gas station and skip the whole field (A.I. characters will pop up with you). If you get down the A.I.s should pop up and help (you can also take a break to pop up if someone else made it over).