Instead of pressing X when you slidetackle someone, press TRIANGLE for a FLYING SLIDETACKLE.
*Note: Doing a Flying Slide-tackle runs a higher risk of drawing a penalty than a regular Slide-tackle, but it hits harder. If you're going to use it frequently, turn off penalties.
As you are running foward press Triangle. When the red par is filled press back (or Down on the D-pad) and let go of Triangle at the same time.
Thanks to Revolution reader Jason Sommerfield!