We're working out what we think - we'll add our view here!
Mountain Equipment says:
A hybrid insulated mid layer that is ideally suited to those needing to move fast over technical ground.
With its exceptional moisture transfer and high warmth-to-weight ratio, the POLARTEC® Alpha® core is the current state-of-the-art in breathable insulation. Allied to proven Pontetorto® Tecnostretch fleece, the Switch is an essential piece for active layering.