Strapping is done to stimulate blood flow to the muscles and therefore improve muscle tone. I've not done it myself but it involves gently slapping or banging the muscles in the neck, shoulders and quarters in a steady rhythm so the horse anticipates by tensing the muscles. Traditionally a wisp of twisted straw is used but you can get leather pads made for it.
I'm sure people do still do it as when done properly it can be very effective but it's definitely not as common as it used to be. As I said, I've not done it myself and haven't seen it done so only have a very limited knowledge of it. I'd be really interested to hear from people who do it!
i do it to my mare when grooming but not very hard and i use a fluffy soft mit. brings a fab shine to the coat and gets some dust off them. i only really do it on the quarters.
i used to have to do every horse as part of the afternoon grooming, when i was at Leslie Law's yard. tbh i think it makes horses bad tempered... they used to hate it. they'd see me coming with the grooming kit and present their bums.
if you do it firmly enough to actually stimulate blood flow and make the muscle tense, then they wonder why on earth you're slapping them. i never ever do my own horses... a good groom and massage with a cactus mitt is much more welcome to them, and they look forward to it.
you do it on the neck, the flabbyish bit of muscle behind the shoulder/above top of leg (it's shaped like an onion) and on the quarters.
seriously overrated imho.