My understanding has always been that pumping draws in and continues to draw in oxygen rich blood as well as lymph fluid until the penis expands to meet the volume of the tube over time. Although, the pressure on the erection diminishes over time as the vacuum space between the top and the lymph fluid expanded base of the penis shortens creating less vacuum pressure at the base than at the head.
Clamping works in a completely opposite way trapping in almost all blood supply expanding the chambers and creating a controlled priapism where some blood can flow in but none can flow out. Clamping, unlike pumping, does not allow full expansion of the entire penis. In fact, the area the clamp is worn to the area just behind no expansion takes place. This can create a severe baseball bat looking erection if not used careful in conjunction with controlled pumping.