The book and the film didn't really tell you much, other than it was sort of round and bounced along the water. The trick was to give it back-spin and get the dynamics of speed and height for release spot on.
Once you've seen one (they've got one at Brooklands, RAF MUseum, Cosford and probably elsewhere) and you figure out the spin trick it is all so bloody obvious how it worked - but like everything else that is beautiful in its simplicity, somebody had to think it up in the first place, so everyone else could say "I could've done that!" - Think catseyes. And, of course, Upkeep needed a decent grasp of maths.
One of the aircraft that went in en-route didn't explode and the Germans had the whole thing reverse engineered in a couple of weeks. They even improved the design by fitting it up with rockets to put the spin on. I've seen a film of that one being tested and, oh boy, did it zing along the water.
Why they were never developed for anti-shipping use I don't know - maybe some forummers can enlighten us.