first a pump track. Is track kinda of like BMX track but with everything tighter together. The track is designed to be ridden without pedaling at all. You gain and maintain speed by "pumping". Pumping is a very refined version of pressure management where you are pulling up hard on the uphills to make your bike weightless and pushing down as hard as you can on the downhill to make your bike heavier therefore propelling your bike without pedaling.
this video shows VERY advanced level pump track riding.
http://www.youtube.com/watch?v=dR2jNIY0Uyw&feature=relatedthis me going uphill on a BMX track to practice my pumping skills, I pedal into but as soon as I hit the rollers I begin pumping as maintain my speed going up a hill.
pump tracks are the reason why MTBing is by far the best cross training for skiing. It teaches you proactive and not reactive movement that translate directly over to skiing 3d terrain. You can literally accelerate and jsut as importantly decelerate on tranisitons on ski if you leg pumping skills are up to snuff. Also the limiting factor for most stronger riders singletracks is there pressure management skills, the better those get the faster you can go.
Pump tracks by definition "flow" if they did not flow you would have to pedal them. When a trail flows it means the turns and up and downs are smooth enough and gradual enough that you are able to maintain nearly the same speed though most of it. Usually with very little braking and pedaling. Features that make you brake or pedal break up the flow. the better the rider gets at pump tracks the more trails will flow for the rider.