cmurder nailed it on this one.

Pin down and pin up mean nothing, it all comes down to your PAP and how far you put the pin from the pap with relation to the angles you create on the ball.

For example, I am a very high track tweener, I'm 5 1/2 over by 1 up on my PAP.  If I ever drill a ball with the pin below the fingers it is greatly increasing my angle to val thus decreasing my motion downlane.  So much in fact that it defeats the original purpose of a "pin under" drill.  These types of drillings for me go very very long with little to no backend.  IE a burnt second shift for house shot ball.

Pros and others talk about pin under and over because they have similar PAPs 4 inches or less over and 1/2 inch or more down.  This is a very different track from what someone like me would throw and therefore their idea of pin over/under is greatly different from what matches up for my game.

A low track bowler's equivalent of a pin down drill for me would be pin directly above the fingers.  

A low track bowler's equivalent of a pin up drill for me would be the pin 1.5+ inches above the fingers.

Always always ALWAYS measure you drillings based on your PAP or even your Track.  

My best ball for Shark is a Rogue Cell with the pin 2 inches above the ring finger and 1/4 inch right.  That is quite the opposite of what others would call a proper "pin down" long oil drill.  But thats because it's what works for my game.

Hope this helps.