Before the F-35 can go into service, it needs an ejection seat, and to make sure the seat is up to snuff, BAE Systems has been firing it out of a bunch of 600 mph rocket sleds.
The new ejection system includes a seat with a rocket pack underneath, plus an "emergency transparency removal and structural severance system," which is a small explosive designed to blow apart the aircraft's canopy to clear a path for the pilot. A mere three seconds after the ejection handle is pulled, the pilot finds him or herself safely away from the plane and descending to Earth by parachute.
It's not really feasible for BAE to test this stuff out using an F-35 Lightning II in-flight, since if it works, that's great, but then how're you gonna get someone back into the plane to land it? That kind of thing gets pretty expensive after a couple tries. So, the way to go is to just bolt the nose of an F-35 onto a rocket sled that can fly down a track at over 600 miles an hour, and launch your ejection seat out of that: