The best way to divide a grass from a pot is to take a sharp knife and cut it into a couple of pieces. I've done this quite successfully. If it's a very large pot, you can go for more pieces.

Now grasses dug up from the ground -- that's another story. These we divide with an ax!

Here is a link that might be useful: Ornamental grasses - planting and care tips

Indeed, heavy equipment needed for large clumps! Just dug up a rather large patch today and divided it into 5 8" sets. started with a sharp spade, which wouldn't come close to splitting it, it was so dense. Ended up using a v tooth saw. No harm done, you really can't get too brutal with these. A tip though, is to wait until after the active growing season is done, cut them down to 6-12" and then divide for best results.

Chop it up at will, it will do just fine

I tried dividing mine a few days ago with a spade......i was not successful. May try my sawzall.