You can see the piston shape here if it helps you to imagine how the outer edge is raised to effectively "Funnel" the mixture to teh middle as it reaches TDC.
If your pistons end up down the bore when sat at TDC, the tops have had a lot removed from them. A Std squish has teh piston 20thou above deck height.
*
Edited to say, thats a crap picture actually, too much glare
