Batter's Interference
There are three ways a batter can commit batter's interference. He can interfere with the catcher making a throw to retire a runner in the act of stealing a base. Second, he can interfere with a runner who is stealing home. Sometimes these two overlap. Finally, he can commit backswing interference (also called "weak interference"). This happens when the batter swings and, on the follow through, his bat hits the catcher or the catcher's glove. Each of the three case is handled differently, so we'll take them one at a time.
1 - Interfere with a catcher's throw
When a base runner is stealing and the catcher comes up quickly with a throw to attempt to retire the runner, the batter cannot in any way impede the catcher's effort – either intentionally or unintentionally. If he does, then the batter is out, the ball is dead, and all runners must return to their time-of-pitch base.
The most common steal-and-throw scenario is a runner stealing second on a pitch (as opposed to a passed ball or wild pitch). Sometimes you see a steal of third, but not very often. So on a steal of second base, you don't often have interference because the catcher has a clear line of sight to second base.
But sometimes you do – for example, when the batter's swing pulls him off balance and he steps onto or across the plate. If there is a runner stealing second when this happens, you can very easily have batter's interference. When stepping over the plate, the batter can easily bump or otherwise impede that catcher's attempt to throw down to second. That's interference.
Note: If you see batter's interference, but the catcher gets the throw off anyway and succeeds in retiring the runner, then ignore the interference. By Rule 6.03(a)(3), if the catcher retires the runner, then, in effect, the interference never happened. On such a play, then, if other runners were also stealing when the interference is waved off, they get to remain at the base they stole.
We said a moment ago that most of the time the steal is of second base. But not always. Sometimes you get a base runner stealing third, and this is the case where you really need to watch for batter's interference. That's because the throw to third often requires the catcher to throw across the right-handed batter's box. And that's where the batter is standing (if he's right-handed). And you can't expect the batter to simply disappear.
Now, this gets a little bit tricky. There is a common misconception that if a batter remains in the batter's box he cannot be called out for interference. This is not true. The batter's box is not a safe haven. But, as we said, he can't be expected to disappear, either. Add to this that a play on a steal of third happens so friggin' fast that the batter may not even know a play is on until the ball goes whizzing by.
And let's not forget about snap throws down to first base to catch a runner leading off too aggressively, or who may not be paying attention (yep, sometimes they catch runners sleeping). You have the same issue here as you do with throws to third, but instead it's with left-handed batters.
So, with all of this fuzzy grey zone, what's an umpire to do? Well, the wording of 6.03(a)(3) is important here: The batter is out if he "… interferes with the catcher's fielding or throwing by stepping out of the batter's box or making any other movement that hinders the catcher's play at home base." As directives go, you can't get much broader than "any other movement." Batter beware.
The emphasis is important because "any other movement" covers a lot of ground. A lot. So the message is, give the balance of judgment to the catcher. Sometimes a bad throw is just a bad throw and you have nothing. But if the catcher's throw gets disrupted in any way, regardless of intent, you've got to call it.
But again (for the third time), you can't expect the batter to simply disappear. You have to watch and judge for yourself whether the batter made "any other movement" that hindered the catcher in any way. This is a judgment call, of course. Generally (generally), if the batter remains still in the batter's box and makes no movement, then he is protected from interference. If it were me, though, I'd duck. But that's just me.
Here's where it gets even more tricky
The batter's interference play that I believe causes the most arguments is not on a straight-up steal. Instead, it's when the catcher mishandles a pitch (or is handling a wild pitch) with runner on base. The ball is on the ground and runners are in motions and the catcher is diving or grabbing for the ball; at the same time the batter is dancing out of the way while trying to avoid interfering, and in doing so he instead interferes. That's interference. "But I was trying to get out of the way," the batter protests. You're breaking my heart, son. You're out.
2 - Interfere with a play at the plate
When a runner attempts to steal home, the batter has to make an effort to get out of the way of the play at the plate. This can be tricky if the runner is stealing on the pitch, as with a suicide squeeze (which is rare, but it does happen). A play like this happens very quickly and even you, the umpire, probably don't realize what's happening until split seconds before it blows up right in front of you.
More commonly, interference with a play at the plate happens when there is a runner on third (R3) and then there's a passed ball or wild pitch and R3 tries to score. You have the catcher scrambling for the ball, the pitcher running in to cover the plate, and you have R3 barrelling toward home. Get position and watch like a hawk.
You have a similar situation on a sacrifice double-steal. That is, with fewer than two outs and with runners on first and third (R1 and R3), R1 will sacrifice himself on a lame attempt to steal second just to draw a throw so that R3 can then steal home. You don't see this too much at higher levels (16U and above) because the fielders are good enough to defeat the play with fast, accurate throws. But at younger levels you see this all the time. Again, watch this one carefully because frequently the defensive will fake the throw to second but instead hit a cutoff fielder, who then makes a play on R3 at home.
3 - Backswing interference
When a batter swings at a pitch and the momentum of his swing brings the bat around and hits the catcher, or more commonly, the catcher's mitt, this is backswing interference. It's sometimes referred to as "weak interference." When backswing interference happens, you must kill the ball ("Time!") and return runners (if any are in motion) to their time-of-pitch base. However, no one is called out.
Rules Variation: High school (NFHS) rules differ on this point. NHFS rule 7-3-5c penalizes backswing interference (FED uses the term "follow-through interference") when it interferes with a catcher's attempt to retire a base runner. See NHFS rule 7-3-5-Penalty for more information.