A presta valve is a little poppet valve that lets air into the tire then pops back and won't let it out. The gauge on your tire pump is actually measuring the air pressure in the pump hose, not your tire. When you operate the pump enough that the air pressure in the hose is greater than the air pressure in your tire, the poppet valve opens, the air pressure in the tire and in the pump hose equalizes, so the pressure gauge on the pump reflects the air pressure in the tire.