The POH of a Cessna T210N (which is a retractable-gear aircraft) states that the hydraulic pump will run 1 - 2 times per hour to restore system pressure bleed down, and whenever the gear up / down lever is repositioned for the whole gear cycle and 1 - 2 seconds more, to restore system pressure. Also, the emergency hand pump can only extend the landing gear, but not retract it. I think it's the same for other RG's (such as the C177RG, C172RG and C182RG).
Now to the question: First, why does the pump need to run all the time while the gear is moving ? Since there should be no pressure loss. Second, why can the emergency hand pump only extend the gear but not retract it - I thought all the emergency hand pump does is building pressure as the electrical hydraulic pump does, and gear position is selected by the gear lever.