what is it? can it be prevented? is it a bad thing? I'm assuming it is normal?
I read post with this issue often, I'm just curious.

Defueling refers to when an aftermarket chip cut back it's power during a shift, or when slipping the trans. This is a good thing for most, and good if you have a stock tranny. Most plug in type chips do this, most programmers do not.

Your stock truck defuels a bit, it pulls out power when the shift is happening to help the trans. When you stick a box type programer on top of it it will follow suit with the factory programing and some boxes will pull out more or less or can be adjusted how much they pull out. Then there is the defeuling that can happen when a box detects slip or high egt's ect. They will pull out fuel(power) to be safer to the vehicle.

You need to reflash the factory tune to change its defuel settings, some aftermarket programs defuel more or less than others.

Defuel=lessening the power output=saftey feature

I think my truck is starting to "defuel" now after a recent history of boosted launches and power braking, does this mean my stock trans is slipping c-1 / c-2 causing the defueling? Not as much black smoke now than before... typical of defueling? Ive been running stage 3 using the Accelerator... '02 D'max LB7

My Banks has been defueling alot lately, If I stay in higher power levels too long, tranny will slip and defueling happens. Also, if EGT's get to high it defuels. Dont think thats ever happened though.

I think you guys are experiencing tranny slip more than defueling...
Jason