It's hard to describe--except to say that it feels like the pad is vibrating against the rotor. It's not the ABS engaging type of feel. It happens sometimes when the car is going 30-40 mph, slightly downhill with light/moderate braking (to control speed). You can feel the resonance and it sounds like the car is 'groaning'. Does that make sense?
It sounds like you have warped or scored rotors. That's common on the 850 because of the weight. It happened all of the time on our R. When you drive it hard or are heavy on the brakes, then it does it for a while. Then it'll go away until you are hard on the brakes again.
We upgraded to slotted rotors (additional cooling) and a slightly more aggressive PBR pad on our 854R project car and the problem went away. You may want to consider a similar setup.
Our setup for your reference was the PBR pad and Powerslot rotor from IPD.