Good question!

It’s a bit hard to tell, because as far as I can tell the phrase “forfeits the king’s grace” only comes up with reference to Saxon law on rape. 

My best guess, from a mention in the writings of Sir Francis Palgrave, is that it has to do with the king’s prerogative to show mercy:

If my surmise is right, this would suggest that, if a man threw a woman on the ground against her will, the king was not allowed to reduce the penalty for the crime. Think of it like the medieval equivalent of a mandatory minimum sentence.