The best way for me to sit ANY canter, is to pick my feet up just barely off of the stirrups. I can't grip with my knees, I can't brace against the stirrups, and I have to force myself to breathe. When I am in a lesson arena I can turn into myself a little bit and trust that the horse isn't going anywhere. Toes up (remember to sink into the heel but don't touch the stirrups!), relax the belly (I have absolutely no control over my back core muscles, but my stomach muscles have a tendency to stay tense and I have to force myself to let go), exhale! (boy do I hold my breathe bad haha)! Relaxing is much easier when I am not bracing against the stirrups, which I do quite a bit on a smaller horse with their itty bitty daisy cutter trots and canters. Larger horses take such big, leaping strides, it actually feels slower to me, even though the ground coverage means I'm moving faster. I have more time to breathe with the movement of a larger horse.
And yes, for some reason sitting through those huge strides works the core so much more. I will hurt after a few strides if I am not used to it in a while.