TDC

From MopedWiki

TDC stands for Top Dead Center, or the point where the Piston is farthest away from the center of the Crankshaft. You can find TDC by putting something long and skinny (like a drinking straw) into the spark plug hole and turning the Flywheel until the the piston pushes the straw out. When the straw is pushed out the farthest, the piston is at TDC.

Paraphrased from this post on the forums.

This is a neat timing tool that can also help find TDC.