Impact shredders work by using a blade to break apart the branches as it hits them. Crushing shredders have a metal cog or wheel that pulls the tree branches - or whatever you are putting in and then crushes it against a plate to break them apart.  The impact shredders are more common, cheaper, and lighter in weight but are also louder. Meanwhile, crushing shredders are less popular, more expensive, heavier, quieter, and they can also pull the branches through without you having to feed and push them in.