OK, here's the deal. Maybe this will make it easier.
When your piston is moving up in the cylinder it squishes the gas/air mixture in to a smaller space or volume. You want spark when it's all squished as you get more power out of the explosion. Most Puchs are set so that the spark fires at around 1.5 mm before the piston reaches the very top of its travel or Top Dead Center (TDC). It sparks just as the piston is reaching the top and everything is super squished. When it sparks, all hell breaks loose and pushes the piston back down to get more gas/air mixture and do it again. If you're timing is set too late, then the spark might be firing when the piston is already on the way down. Not cool. If it's sparking too early, then it kinda slows the piston down as it is on its upstroke. Does that help at all?