it was to get past torpedo nets hung around ships while anchored
The main initial target was Tirpitz, laid up in Norway in a fjord
The idea was to bounce the bomb over the nets, then have it sink below the ship and hole it low down where the armour was thinner.
A torpedo would hit too high - on the thickest part of the armour plate
there is apparently a training video in the war archives of 618 Squadron bowling rubberised Highballs down a railway tunnel in Wales. Its mentioned several times on the internet and theres a thread somewhere on pprune from a chap who has seen it, but it seems not to have leaked onto the internet