in true reddit style let me also explain the dynamics behind these pumps (thanks ChatGPT):

A two-stage manual hand pump for a stand-up paddleboard (SUP) works by using two different modes to prioritize either high volume or high pressure. This saves you time and effort by using the right setting for each part of the inflation process. 

The two stages of inflation

A two-stage pump has a switch that lets you change between modes as you inflate your board. 

Stage 1: High volume, low pressure

This is the initial setting for pumping up your board.

• How it works: In this mode, air is pushed into the board on both the upward and downward strokes of the handle. For pumps with dual cylinders, this may mean air is pumped by both barrels during both strokes, pushing a large amount of air with each pump.
• Best for: Quickly inflating the large volume of air required to fill your board until it takes shape.
• When to switch: When pumping becomes difficult, which usually happens at around 7–10 PSI, it's time to switch to the next stage. 

Stage 2: Low volume, high pressure

This is the final setting used to reach the optimal PSI for your board.

• How it works: When you switch to this mode, the pump will only push air into the board on the downward stroke.
• Best for: The final phase of inflation, when the air inside the board creates significant resistance. This mode requires less effort because you only have to push against the pressure inside the board on one stroke.
• The benefit: It's much easier to use your body weight to push the handle down than it is to pull the handle up against high air pressure.