We build a tower of draughts near the edge of a smooth table. Then Alex quickly slices the ruler through the bottom of the tower by sliding it along the table. With some practice Alex was able to knock out the bottom draught without toppling the tower. The quick push needed to knock the bottom draught out is too small a force to overcome the inertia of the whole tower.
The next stage for Alex would be to practice trick of whipping the tablecloth out from under some plates and glasses ✨