Baseball, by the Rules
The Runner Gets Back
A runner takes a lead off from their base. Maybe they're already planning on attempting a steal, and maybe they've even actually started running on a previous pitch that was fouled off. Maybe they just have a good reputation as a base-stealer, and the pitcher is being vigilant. Alternatively, maybe the runner is just minding their business, and the defense just wants to go after them1.
No matter what the circumanstance, instead of pitching, the pitcher throws to the base in question, hopefully managing not to balk along the way. But! Their efforts are in vain; the runner gets back to the base quickly enough to avoid being tagged out by the fielder. The runner does still have to be careful, here— the ball is still live, unless time is called2, and the runner is still in danger of being tagged out as long as the ball is in the vicinity of the base. Assuming there is no funny business, the ball eventually gets thrown back to the pitcher.
The pickoff attempt counts as a disengagement. Once the ball is thrown back, the pitch timer is reset, giving the pitcher a new 18 second countdown to work with, and the whole thing resets.