That's called reverse push. The function is to make the plane slow down as soon as possible after landing and shorten the landing distance. Its principle is to change the direction of engine injection from backward to oblique forward! There are two kinds of reverse material return, baffle type and grid type.

Generally, it needs to be pushed and pulled, and it is normal for a small plane like CRJ to push and pull. When the taxiing speed of the aircraft drops to 80 knots (about 40m/s), close the thrust reverser. ?

If the thrust reverser fails, it can be reserved without thrust reverser, but when runway pollution, plateau and short runway affect the braking efficiency, the plane will not fly. Moreover, it is very troublesome for the crew to carry out supplementary procedures to reduce the limited weight of wet runway/obstacle-crossing and the corresponding speed of V 1, and to understand the airport runway situation and form a plan to stop taking off and landing.