How does this work?

[trajectory]

On opening the pump arm, there's a point at which the vacuum in front of the pump seal will vent, either by the seal clearing a slot or a drilled hole, allowing the next lot of air forward of the seal to be compressed on closing

Thanks, I thought there would have to be a vent....those seals must be pretty special to cope with running repeatedly over it, must be very resilient even running over a smooth apperture,