PWR keeps the revs high for maximum acceleration and engine braking - i.e. doesn't change up 'til nearly redline even when not flooring it.
HOLD will pull away in second and keep the revs as low as possible in an attempt to reduce wheelspin on low traction surfaces (often toted as winter mode on other manufacturers auto boxes). This is also very useful with low range when off roading as low-second is low enough for most UK off roading, saving 1st for when it's really needed - steep climbs and descents.
In the centre position it behaves like most modern auto boxes, adapting the shift points to current engine loading and accelerator input.
Alaric.
