F1 drivers sit in front of the power unit which has its gearbox attached at the very rear of the car. As such, it is susceptible to damage if hit from behind or if the driver slides backwards into something solid.
When a driver is close behind the car in front it is often said that he is sitting "on his gearbox".
[caption id="attachment_5739" align="aligncenter" width="640"] Gearbox : racecar-engineering.com[/caption]