Equip a weapon and press B to jump. Right after jumping, switch weapons. This will cause the jumping action to be repeated, so you can double jump. In this second jump, if you change weapon again, you will jump another time. You can repeat this as many times as desired, allowing Rico to climb any building or gap, provided you start with a forward (not stationary) jump. If you do not manage to do the trick, Rico will simply fall. If you do the trick too quickly, Rico will either remain still or go horizontal. It is best to alternate between two weapons rather than to cycle along, and to have more than two weapons in case you change to unarmed or do not change weapons at all. This trick allows you to get huge base jumps and is thus a useful technique if you are stuck in the jungle, miles away from any road.