In picture "A" above you can see with the wrist bent back that part of the weight of the ball will be resting on the base of the thumb.

在图片“A”你能看手腕向后摆动时， 一部分的球的重量会移向拇指的下部。

So Tom unwound the thread from one of his needles, and each boy pricked the ball of his thumb and squeezed out a drop of blood.

31于是汤姆拿出一根针，去掉了线。两个孩子各自往大拇指上戳了一下，然后挤出两滴血来。