The function of the balancing spindle is to prevent those dreaded toilet flush, scalding shower scenarios. On earlier tub/shower valves, when the cold water had a pressure drop, from a flushed toilet for example, the temperature could potentially become scalding. This is because the ratio of hot and cold gets thrown off due to the lack of cold water feeding the valve briefly. A newer pressure balanced tub/shower valve will not allow this situation to happen. It has an internal "spindle" that can freely glide back and forth, and compensate for any pressure differentials. Eventually, just like anything else plumbing related, this spindle can go bad, and usually get stuck. When it gets stuck or goes bad, one of two things usually happens; there will either be little to no water coming out of the valve, or it will vibrate your pipes so bad you will think there is a jackhammer going off somewhere in your house.
If your shower is not performing the way you'd like, or it is performing a little too well (making music in the walls), give us a call so you can start enjoying your shower once again!