Back paws.
This is the harder half of the board, and most places that sell one do not mention it. Front paws are a fairly natural digging motion. Getting a hind paw to do the same thing on purpose is a real training exercise, closer to the cooperative care work a lot of owners are already doing than to anything front-paw scratching resembles. Expect it to take several short sessions, not one.
Why the hand target matters
A dog can see and smell the board directly, which is exactly why front paw scratching comes easily and hind paw scratching does not. They cannot see their own hind feet or the board at the same time, so there is nothing to aim at directly the way there is for the front paws. The hand target gives them something to orient the whole body around instead, and the turn that follows it is what actually lines up a hind paw with the surface. Skipping straight to physically guiding a hind paw onto the board tends to build a habit of waiting to be moved, rather than a dog who offers the movement on their own.
When it stalls
The turn is fine, the paw never lifts
Go back to rewarding smaller pieces. A weight shift onto the hind leg, without a full lift, is still worth marking. Most dogs that stall here are being asked to jump a step rather than being under-rewarded for the step they are actually on.
They sit down instead of turning
Sitting is usually a dog checking whether that gets rewarded instead. Simply not marking it and waiting for the head turn again, rather than repeating the cue, tends to resolve it within a few reps.
One side works, the other does not
Completely normal. Most dogs have a side they turn more easily. Keep practicing the weak side in short separate reps rather than alternating within the same session.
Still stuck after several sessions, or the dog has stopped engaging with the front paws too? See the troubleshooting guide.