What people refer to as "velocity" porting - is when they try different port shape alterations using plasticine/clay to build up certain areas (ie port floor ) and discover (on the flow bench) that they can increase the velocity without losing the cfm of flow by replacing the clay with weld metal or epoxy .
The proper name would be "correction" porting and that's what motoman is basically doing . Sometimes a port can be too big or poorly shaped and angled as stock so added power can be found by altering and reshaping things . Velocity = throttle response and punch - hog a port out too big and you end up with a boggy engine that won't pull the head off a match stick .
Here's some info with porting ideas on pitbike heads :
Mini Dirt Bikes & Pit Bikes Forum : MiniRiders.com.au - Porting!
BBR welds up the port floor on DOHC Daytona heads so even the most expensive heads can have poorly designed ports :[QUOTE : Porting the head and welding and reshaping the port floors resulted in an additional two ponies and a boosted rev range.]
2006 Title-Winning BBR 119 Test - MotorcycleUSA.com