I posted this for someone's clarification on the Dag boards. I figured it would fit well here. If anyone has commentary on how they do these grips or has a different terminology than I do, please let me know! This is just from my understanding.
Grips!
Hammer Grip:
The most common of grips, useful for many, many things. Use it, know it, love it. It's an excellent transitional grip and an excellent default grip.
Rogue Grip (Modified French Grip):
My favorite grip at the moment and used by many top-end fighters. It's superior for stabbing and punch/handle blocking (it helps protect your outer arm due to the angle). It also allows quicker slots and arm-chops, but can limit your range and strength on straight chops if you aren't willing to correct and adjust your grip on the fly.
Flat Grip:
Kind of like Rogue taken to the next level. It's a great grip for throwing slots, sweeping shots (like, where your blade moves like a windshield wiper), and for throwing certain "wraps" and crosses. I say "wraps" because, thrown from flat grip, the shots aren't as wrappy, but land with much less wrist torque. I like it for when I feel like high-crossing.
Pommel Grip:
Favored by some for range and angles. I don't use this grip often, but it does give you that little bit of extra range, if you need it. Beware shot lag and pommel breakdown if you haven't built your pommel super-tough.
Saber Grip:
I don't like to use saber grip. I've injured my thumbs using this grip. Don't block in this grip, for sure. Again, not recommended, though you do get fabulous control. Instead, use...
Modified Saber:
Taking your thumb over so that it isn't directly in line with the grip, so that you're alllllmost rogue-gripping, but lower, on the handle, and you'll be in Modified Saber. This gives you nearly the same control as saber with less likelyhood of breaking your thumbs. Good for when you need range and tip control and don't feel like using something else, I guess. I use it sparingly, I suppose.
Anyway, enjoy!