Putting a male slave in a very small cage is an effortless and effective form of discipline.
He is forced to kneel or squat disallowing him the dignity of standing. Severely restriction his ability to move will produce restless discomfort.
A male slave can be kept in a tiny cage for quite a long time. Over time his muscles will begin to cramp and ache. Rendering the slave temporarily useless may not be an important issue for its Mistress Owner.
The psychological experience of being locked in a small cage can be deepened by putting the cage in a dark, empty room.