Hi Clammy,
This is the basic principle for all transmitters. No transmitter on the market today has an on/off button. Instead what you do is that you sample the pressure at a slow speed, not to drain battery. When you detect a change in pressure you start to sample fast and turn on the transmitter. If there is no change in pressure for some time then you turn off the transmitter and go back to slow sampling. I never heard about anybody complaining. The good thing is that it's all automatic.
The Uwatec pressure measurement is very good. Actually, you can count every breath the diver takes thanks to the high resolution analog to digital converter. Uwatec needs a sensitive measurement for to the diving model and workload estimation. Other manufacturers uses 8 bits with a few tricks, that's cheap and simple.
Niclas