Momentum Boost is a trick commonly used by experienced players to close Long Jumps or to pass through Arrow Walls. In many advanced minis, players have to gain velocity in any way possible to jump farther than usual. There are several tricks to pull this off.
You can gain instant velocity by jumping off a wall as shown in the picture, many variances such as jumping from the ceiling are often available.
You can gain extra distance to accerelate yourself by closing the gap between two separate platforms. (as shown in the picture)