Most O gauge couplers (The big ones called lobster claws by some.) have a tab that hangs low just above the center rail. Uncouplers are electromagnets. You do your best to line up the tab on the coupler with the center of the uncoupler. When you push the uncouple button it sends a current to the uncoupler making it magnetic and it pulls the coupler tap towards the track which pops the coupler open. You'd use them in places you'd want to leave cars behind, so switching yards, spurs, both ends of a siding, things like that. As for location, depends on where you want to leave cars. Experience has found you don't want them right up next to a switch.
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