The slightly sickening feeling that comes shortly after being hit or kicked in the balls. At first the blow may not seem too bad but then you feel the effects in the pit of your stomach.
After-tap can occur up to 1 minute after the initial sack tap or bell tap.
"Ah man I thought the worst was over but now comes the after-tap!"
or, following an attempted sack tap
"Aha, you didn't get me that badly... oh shit wait, here's the after-tap"