What Overreaching And Forging Actually Are
Overreaching happens when the hind hoof comes forward and hits the heel or back of the front hoof before the front hoof has left the ground enough. Forging usually refers to the sound of the hind toe hitting the front shoe or hoof as it breaks over.
Both are about timing, stride length, and balance. The hind feet are catching up to or overtaking the front feet in a way that is not coordinated.