Like Qtrswade, I also learned this from a profession horseman/ trainer. Just wanted to add that when you do this manuvour you grab your rein as near to the bit as possible and place your horses nose on your knee, there must be no tension in the opisite rein, it must be loose or you have a bad two rein stop which your horse will brace against.
Don,t release till your horse is totally relaxed. You can also add to it by gently tapping with your foot to disengage the hindquarters.
I do this with my horse all the time, just to put him in his place when he is being disrespectful and not listening. Its a good wake up.
Also be careful as in the right conditions, you can take your horse off their feet.
There are loads of videos on you tube that demonstrate the correct way.
Have a go and let us know how you get on.