For more examples, click here though a disclaimer is in order, as due to the way that MLB tags pickoff plays, this set includes both catcher pickoffs, as well as plays in which a pickoff is attempted, but the runner is called safe. Maybe those get overturned on replay in a manner in which the clips don't capture? Whom's to say?

A Runner is Picked Off

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. If they're fast enough, and if the runner was sufficiently fooled (or was too far from the base), the ball will get to the fielder in time so that they can tag the runner out. The runner is thus picked off.

The tag rules here are the same as they are pretty much everywhere else. The fielder has to have the ball secured, and then has to tag the runner while said runner is not touching the base. The fielder cannot fully block access to the base, unless they already have the ball. Most pickoff plays are extremely fast, which means it's hard for a fielder to get into an obstructive position, unless they were set up in that position when holding the runner on. But if they did that, they could be called for obstruction, thus making the runner safe by default.

Sometimes, you will see a runner take off for the next base as soon as they see the pitcher move. However, if the pitcher's first move was to throw over, that runner is going to be toast, unless they are extremely fast (or the defense screws up). The fielder will take the pickoff, and throw to the next base, where the runner will almost certainly be formally tagged out. This particular move is scored as a combination of a pickoff/caught stealing, and you will see it written out as "[Runner] was picked off and caught stealing."