On Wing Coasters designed by Bolliger & Mabillard (B&M), keyhole elements are common. These elements feature both headchopper and footchopper effects with the train passing through an object only just big enough to accommodate it.
The Keyhole Effect
Wing Coasters are unique in that riders sit in pairs on either side of the track meaning that riders are completely suspended in mid-air with nothing above or below them. The keyhole effect occurs when the roller coaster train passes through the centre of an object, giving the illusion that the train and its passengers have just enough clearance to fit.