They will send out one or two new stems from the node where you cut them. Cutting from the top is a good thing to do if you have a tall tank and you're trying to get your plants bushier as they get taller. However, if you have a short tank, it's better to just pull up the entire plant, roots and all, cut off from the bottom, and replant the tops, because they are much prettier and healthier. If you keep cutting from the top, the bottoms will wither away eventually, because nothing lives forever. Keeping the tops means you have the prettiest part of the plant. Of course it's a lot more work, but you get used to it. When I have very fast growing stem plants, I alternate between the two methods, so I don't have to uproot the entire tank at every pruning. And, of course you can cut from the top and keep both the tops and the bottoms, but with fast growers you will run out of room soon and have to start choosing what parts to throw away.