If you are interested in having this done, come to the medical centre and the Back on track will explain the procedure in more detail.
In essence there is a small injection of local anaesthetic, then a needle is inserted about 2cm into your arm, the resting pressures are measured. We then get you to provoke your armpump using a handlebar rig we have, we then remeasure the pressures.
Typically it is better to have this done when you have finished racing for the weekend.