Trying to cut down a branch high on a tree with a chainsaw is a good way to gamble with your digits. If you value your thumb, perhaps you should find a better way to cut down high limbs. Might I suggest this ingenious hand-powered chainsaw? It's the links from a chainsaw with 50 feet of string on either end. There's a weight at one end, which you lob over the branch in question, and then you simply pull back and forth on the strings to start sawing. Check out a picture of it in action:

[Product Page via Gadget Lab and Hardware Aisle]