I have used the usual thin nozzle at the end of a hose for years, the sort you have to twist the end of one way or the other to make the spray wider or narrower, which takes 2 hands (one to hold the nozzle, one to twist the end).
This spray attachment (pictured) has a squeeze trigger, and the more you push it, the more pressure, and the less diffuse the spray. It has a one handed action, and you can change the pressure/spray instantly.
I can’t begin to tell you how much easier this makes it to wash off a reluctant horse! The other hand can be used to calm the horse, hold the shampooey-sponge, etc.
It’s just a little thing, but day-to-day these little things can make a big difference!