this page is a mess at the moment
I've had the finger of doom jump in and destroy my territory a couple of times, in rapid fashion. The basic idea is a couple of stones single-space jumping on the edge. You can't cut because you're on the edge, and you can't even directly block the lead stone for the same reason (unless there is a another friendly stone somewhere on the edge behind you).
As far as I can tell, you have to block a space down from the finger of doom, which may have already deeply intruded into your territory.
Seems like you have to have an open side for this to work, as a stone on second line would allow you to prevent it. Then again, this length of this extension could have been prevented anyway... so either I wasn't paying attention or I thought something more important was going on.
The interesting thing about the Finger of Doom is the rapidity that it can shoot into territory and cause havoc. Remember, the blocking point is one further down than furthest intruder (a in diagram), and he can move two at a time!
I can't find a page describing this, although there must be one.