Bastinado is the slapping or whipping of the soles of bare feet. It can be very painful but can also be the prelude for incredible tickling when used carefully to sensitize the bottoms of the feet.
In the tickling scenes I've had (both as a lee and a ler) we used wooden spoons, paint stir sticks, and other flat surfaced items slapped against the bottoms of feet. It brings the blood rushing to the area.
It should not be tried by anyone who is not well skilled. If used incorrectly it can actually cut the feet or leave bruising. But when applied by someone who has the skill mastered the victim's feet become incredibly sensitive and the tickling afterwards will drive you absolutely insane.
I was totally against it when I first heard of it. Then I agreed to have it done and found it to be scary at first but the tickling that followed totally drove me over the edge. The person who showed it to me was very careful when he introduced it to me and didn't take it to extremes fortunately. He was very skilled at the practice and helped teach me how to utilize it in play scenes.