In Situ
Chastity

Related
Cock and Ball
Bondage
Total Control
Impaling

Trainer Position

In any sex shops you can by ankle-cuffs (like the Hiatt design), you can find the centre of the chain and loop it around the cock and balls to provide a 'cock ring' that is held in place with a padlock; the clasp of the lock also connects to a chain secured in the floor behind the slave.

The cuffs go onto each ankle so that slave is kept on his knees, attempting to stand up will pull at his cock and balls and there is no chance of it coming free unless the cock is very soft. This is also good for auto-bondage for solo players. The slaves paws can be hand cuffed to the chains that go to each ankle, or to the chain that goes to the floor, as he pulls on the chain he pulls on his balls, he can reach around to the front only to find that his cock is being pulled out the way. You can add to the effect if there is chain from the ceiling to his leather dog collar keeping him in position.
The chain that leads from his cock and balls to the floor 2 feet behind him also strokes his anus as he moves further arousing him or ensures that the butt-plug that you have buried deep in the boys gut, stays there but is massaged now and again. A further addition should a long chain be used, is for the reminder of the chain to be fed up to the slaves collar. This the hand cuffs could be connected to (assumes 2 pairs of handcuffs) the cuffs tug on the chain that now goes from the cock and balls, over the slaves hot butt hole, along his spine to the leather collar.

Advantages

Again if the slaves leans forward it pulls tight, if the slave tries to pull his hands in front, it pulls tight.
He might feel the overwhelming need to do this to touch the leather of his Masters legs as the Master stands in front of him with his cock in the boys mouth. This is a good position for when pissing down the slaves throat or making him work hard at cock sucking. You will enjoy seeing how hard he is working for you. If the chain did not go to the collar, it also keeps the slave in a nice position for a light beating. It is also quite nice for shit feeding by securing his had to the chain in the ceiling, and allowing his hands at the front so he can finger himself.
You can also use this trainer position for his ass training, he could be sitting down on a well lubed butt plug or dildoe as his knees gives out he slips further down, impaling himself on it.
For a while he might have the energy to fuck himself with it, but in the end it he will be taking it all. You might amuse yourself by putting a little lube on his heels to that he ass cheeks do not grip the heels so he cannot use that to prevent the inevitable.