It's a mix of a supple and strong grip, you need to adapt it at each given moment, either bracing if you're expecting a cut, but even during a cut you must keep it both firm and flexible as not be able to retain it's mobility. I have to admit it took me quite a while to be able to use it effectively with a sabre more than anything precisely because of all the cutting and high impact.
I don't know how you grip your sabre loosely enough to power cuts with your fingers and not get it knocked out of your hand at the first clash.
It's a mix of a supple and strong grip, you need to adapt it at each given moment, either bracing if you're expecting a cut, but even during a cut you must keep it both firm and flexible as not be able to retain it's mobility. I have to admit it took me quite a while to be able to use it effectively with a sabre more than anything precisely because of all the cutting and high impact.