Fa-jing it the power source or the internal arts.

It is a whole body explosive whip action, that allows us to create power over a very short distance.

When you see someone do Fa-jing on someone, if they fly back 6 feet, this is not Fa-jing, it's a push.

Fa-jing should cause very little backward movement, but instead a violent shock into their body. 
When you hit someone with Fa-jing, they drop where they stand.