[I tried to find a photo with a woman doing a boleo, but I couldn't... which tells you where I've been dancing...]
From Limerick Tango, this one bears repeating in every class, practica, milonga and blog across the land...
"You don't *do* boleos. Boleos happen." [Limerick Tango]
At the aforementioned recent tango event, unled boleos were being done all over the place.
The difficult part is conveying that instantaneous state of controlled flaccidity or ready relaxation in the free leg, alternating from leg to leg, so that the leader can cause a boleo to happen.