Impossible Pogo Trick
Normally once you have activated the pogo stick you can hold the jump key while pogoing, and you will jump much higher than pogoing normally. Due to the design of the game's engine, you can actually jump a fraction higher than this if you activate the pogo and then press jump before the pogo hits the ground the first time. It only allows Keen to jump a fraction higher than normal, but it is often just enough to access many secret areas in the levels. It also means you do not need to wait for the pogo to hit the ground once before you can gain full power for the jump.
This trick is much more difficult to do with the two-button firing enabled. It was so named by the game designers in reference both to the fact that it should be impossible to do with a normal pogo stick and that it can be difficult (impossible) to do in-game.
Here is a comparison of the normal jump (with pogo stick of course) and the impossible pogo trick: