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In pinball, a safety pin is a pin in the bottom of the playfield, usually lower than and between the two flippers. This acts to bounce the ball back up, should it dart straight down the middle. A lot of tables employ this to take the amount of randomness off of the table, and lean it more towards the skill (or lack thereof) of the player.

Safety pins are usually positioned so that the ball has enough room to roll down on either side of it, so that if the ball were to roll off of the flippers it could still go down the middle. The pin typically looks like a nail in the bottom of the board, with a rubber ring around the outside.

To properly use the pin, I'd suggest doing nothing. Very seriously, if the ball looks like it's going to go straight down the middle hole (and you won't even be able to get a piece of it, and bumping the table isn't your thing), the worst thing you can do is to start mashing the flippers. Doing so will usually kick the ball with the back of the flipper, sending it quickly into oblivion. Wait for the ball to hit the pin, bounce a little (hopefully to one side), and give it a bit of English to make sure it's going to do what you want.

Very few tables have these pins, but the ones that do, add an entirely new element of skill to the table. Most newer games have abandoned this in favor of things like minimum playing times, and other saving grace features built-in to the table to assure playtime value.

I have also been informed that a safety pin (as it is called everywhere I have seen it) may also be called a center pin. YMMV.