As its name suggests, the ring of polymorph control is a ring that grants polymorph control when worn.
If you are wearing (or have eaten) this ring and something causes you to polymorph, then you will be asked "Become what type of monster? [type the name]". You can then choose to polymorph into a monster.
Rings of polymorph control will also allow you to decline a change in form brought upon by lycanthropy.
Polymorph into your own species will cause you to "feel like a new (wo)man!" or whatever species, and cause random change to your scores. There is a chance that this will happen in spite of polymorph control, even if a different monster is chosen.