It happens occasionally, that I come across something I don't know how to do, and I need someone to show me. So I ask Bob for help, because Bob knows this stuff, and he comes over and starts explaining to me. He talks for a couple of minutes, gives me an example or two, and I say "Ah, I see - ok, I've got it, thanks for your help".
Then Bob keeps talking. And talking. And talking. He spends several minutes continuing to explain to me, even after I already understand it. I've tried saying "Ok, I understand now, thanks for your help", but this is apparently not clear to him, and I don't want to waste several minutes sitting there bored while he talks.
How do I get the point across that I want him to stop talking and let me get on with it, without being rude about it? I don't want to offend him, or make him not want to help in the future.
Please answer in the context of Western culture - this is specific to England / America, and I know that definitions of "rudeness" varies from place to place.