Yep, you're right on about "speeding up in rapids". But besides jumping a drop, there's another time you want to go fast. That's when canoeing very spiky drops that are laced with rocks that prevent ferrying. We discovered this in Norway last June. The rapids we ran in our solo folding canoes all pretty much rated technical 2+ to 3. There was no space for ferrying.And rocks were everywhere and often so close that it was impossible to pass through without nailing something. Here, the answer was speed enough to dash quickly from one current to another. It was a different style of canoeing, one I wasn't used to--and not at all like the more open rivers in Canada. As to the "trailing rope", even without a knot a rope can catch on something. I thought the idea of it was ludicrous.
Dock & Launch Systems
URCHIN Portable Anchor