The handle on the roof or up high on the side of a car on the inside that you can hold while going around really sharp turns, etc. but usually never use. When you do use it it is probably during something very dangerous, i.e. drifting around public roads or swerving around deer, and gets its name from the words you exclaim at the times you have to grab it.
"OH SHIT" I said as I grabbed the oh-shit handle when we were drifting around the really sharp turn.