Blocking is a term used in tricking to describe turning horizontal momentum into vertical momentum by punching or lifting off the ground at an angle in order to gain height. Blocking is useful in almost all flips that are performed while moving forward or backward, and therefore is very useful in both tumbling and free running.
A normal jump would involve bending the legs and using the power from the extension to gain height, whereas while blocking an athlete will keep their legs straight, using the spring from the floor and their legs to transfer their horizontal momentum into height.
The physics can be compared to a ball bouncing off a wall - if a ball is travelling horizontally, and hits a diagonal wall facing upwards, the ball's path will change and it will travel vertically. This same principle can be applied to, for example, a frontflip. By planting their legs in front of the normal position, and keeping them tensed, an athlete can turn the momentum from their run-up into height.
The term blocking originates from gymnastics, however in that context it refers to using arms rather than legs for spring, whilst punching refers to using the legs.
- "Definition Of Gymnastics Terms". Drillsandskills.com