As mentioned it is usual practice for 3 year olds,they are backed and do simple training then are turned away to grow up mentally and physically,ready to be brought back into work properlay as a 4 year old.
I see huge benefits with it TBH,they don't forget things,they might need a brief refresher when brought back into work,but usually pick it up again quickly.Sometimes training for a young horse can be confusing and hard work,it takes a fair bit out of them and they can if pushed too hard too soon become sour and develop problems with behaviour.Turning away gives them a chance to chill,not to have to think about stuff too much,and generally be a horse,rather than someones hobby or leisure time.
They usually come back refreshed and keener to be educated.I guess it's like people when they are pushed too hard and pressured they become resentful and frustrated with the whole thing.
That's not to say that you can't do anything with them if you still wanted to,and even if do some ridden work then that should be fine and can still be incorporated with being turned away,IMO anyway.I guess it's about balance,as long as they have enough time to relax and not have to think about too much,then doing some stuff they enjoy shouldn't be a problem,it varies with every horse obviously,but most don't mind doing some low key easy stuff.
There is also the element of the time of year,I find turning away in winter useful from my own POV (and I do it with my horses whatever their age for at least some of the winter).I don't have facilites so doing any work is difficult,and TBH quite stressful.Taking the pressure off me to work them helps me relax and enjoy them more,and so in turn stops my stress transferring to them,which particualrly with youngsters is hugely important.If they sense you are worried or stressed then they will be too.
Sorry that was longwinded,and is only my opinion of course,but hope it was of some help:smile: