I've 'played' with L a little bit. At 45 mph, drop it in L, revs increase but not to an extreme, and as you slow down, it still downshifts about 3 more times on its own. You can feel it some, but you can also watch the tach and tell when it downshifts.
I don't think the computer will let it over rev as I was driving slow once (because someone had to pull out in front of me and then decided to turn on the next street!) I dropped it in L and punched it. It upshifted before I could bump into D.
Depending on your speed and what gear its in (and if its around a shift point), it could really bump up the revs and feel like its about to blow, but I don't think it will. If you were already going down hill and it was in 3rd (low rpms), the speed may have been slow enough for it to go to 1st when you put it in L therefore really increasing the rpms.
Just my uneducated guess.