When you don't let off the throttle between shifts it's called powershifting. I shift like this with both my cars at the track. It is harder on the cars drivetrain but does drop some time. Even if you just let off only a little bit in between shifts in is better than fully letting off. My Mustang doesn't rev up much at all when I powershift..I time it right and shift slam through the gears HARD. I've only missed 1 shift at of over 60+ runs this year. Thats pretty good. Just have to practice...

'00 Harley-Davidson F150
'88 Mustang GT
'93 Mustang LX 5.0
'00 Focus (beater)