I think I finally understand detuning a little better. I found this on the web:
Detuning for carving
Detuning is the process of dulling the metal edge around the tip and/or tail to allow the board to release easily between carves. Without detuning, some boards will feel hooky. However, detuning has the effect of shortening the effective edge of the board, so it is not recommended for carving. After tuning your board, do not immediately detune the tip and tail. Instead, take a diamond stone with you to the slope, and detune only if the board feels too catchy. If the board feels hooky at the turn initiation, detune the tip. If it won't release from a turn, detune the tail. If the board catches a lot on the flats, that is also a tip-off that you might want to detune. Make 1 or 2 strokes with the diamond stone, rolling it from flat to 45º to feather the transition from sharp to dull, for about 10 cm around the edge/tip or edge/tail transition. You can also run a gummi stone at a 45º angle. If you find that you need to detune the tip and tail, you might want to consider more base bevel, which will make the board release easier and obviate the need for detuning. However, detuning is ultimately a personal preference. Other tips:
If you get your board tuned at a shop, insist that they not detune the board. That's because some shops apply excessive detuning, which will completely ruin your edges.
Some people find that a board will naturally detune itself after a few runs.
Boards that are very damp, or that have no taper, or that have a lot of sidecut depth may need more detuning. The Oxygen Proton is one example of a zero-taper board that can be unrideable with a small base bevel and no detuning.
You can buy diamond stones in a butterfly-knife style holder that folds up, which you can take with you on the slope.
Printed on the topsheet of the Burton Speed is the advisory "Detune 50mm @ Tip & 30mm @ Tail", an embellishment most likely added by the lawyers.
If you are racing, detuning will help skid the board at various times on rutted-up snow around gates. However, custom race boards typically have a huge taper that obviates the need for detuning. Sigi Grabner detunes at the tip and tail, and it seems to work for him.