A headchopper is any point on a roller coaster where the support structure of the ride comes very close to the passengers' heads, or at least appears to do so. All headchoppers are, of course, designed so that even the tallest rider, with both their hands up, would be unable to touch the structure; although if a rider exceeding the maximum height does board the coaster it could be potentially dangerous. Headchoppers are most common on wooden roller coasters but are also found on many steel roller coasters.
The inverted roller equivalent is a footchopper. Footchoppers are designed such that rider's legs appear to come close to the ride's support structure, water, or other ride surroundings. Suspended Looping Coasters, such as Vekoma's Mind Eraser, are known for their footchopper effects because of their compact layout. For example Dragon Challenge at Islands of Adventure has many footchoppers, where the rider's feet come within feet of the ride's supports. Vekoma's Suspended Looping Coasters also feature an intense footchopper during an in-line-twist, in which the train approaches a section of track directly below, making it appear that the riders' feet will impact the track if the train remains on that course; but the train undergoes an in-line-twist right before the obstruction, twisting the riders onto their backs as the above track crosses safely over the track below.
On Bolliger & Mabillard Wing Coasters, keyhole elements are present. These elements feature the effects of both headchoppers and footchoppers. The train, which seats riders in pairs on either side of the track, passes through the centre of an object just big enough for the train and required clearances to fit.
Read more about this topic: Roller Coaster Elements, Basic Elements