Note that this trick is patched and will not work anymore in the latest patches.

 

All you have to do is send an unit to atack a brave that is in queue to a training hut. Then, fastly, right click you unit, click on the unit status, whch will lead to the enemy brave status, then click that again. If you did it fast enough, you have now control of the mantle/dismantle switch of that training hut, and you can see the training bar aswell.