not much, if anything will come out if you can clamp down the right spot.
and yes, it reduces the refractory period. I still needed a small break because my cock would get really sensitive, but after 10-15 I was good to go like I hadn't cum in the first place.
if I blow a huge load I can get a roud two going after 20-30 minutes but its a LOT tougher and I'm usually working with a marshmellow cock.