Or until you add it back manually on the fly.
Helpful to lock out on rolling hills when it shifts into 6th downhill, 5th uphill, etc..
Get's annoying when it's up and down for 20+ miles.
Most of the time I just throw it in M6 though.
I use this feature in D religiously while pulling for on the fly gear management via button. On up hill I can hit - couple times down to 4 and she smoothly goes to 4 then on the downhill side go back to 6 or up to 5 to force it to stay in 5. I love this truck. Whoever did the gear button and lighted letters idea needs a pay raise. No need to shift to M to use the + - button feature.