a non mathematical or scientific way to look at Port velocity/. in strickly visual terms..think of port velocity like a rope..you want a nice tight rope.. . but keep all the slack out of the rope gives you best velocity.

another way to visually understand it..is a garden hose turned on just a little bit... with an open end on the hose..hold your thumb over the end of the hose.. let it build pressure too keep it spraying... hold it too tight it backs up.
remove too much thum and just trickles out.