IIRC they made a change in '08 or so, where it reacts faster. But yes, it was predominately FWD, reacting to wheelspin.
From what I understand, while you could think of it as a center diff, the traditional 4x4 terms don't apply, at least on the RAV4. On that setup, the driveshaft going to the rear is powered always--think of the transmission output going through a locked center diff to both front and rear diffs. Then, in the rear diff, a variable clutch is used to apply power to the rear wheels, in a linear fashion.
So, AFAIK these setups really only use two differentials: the front diff, and a complex rear diff which uses a linear clutch of some sort to apply a varying amount of power to the rear wheels. [This linear clutch is what also prevents drivetrain binding on dry pavement, as it is never reaches full lockup.] Depending upon how you want to look at it, there is no center diff; or it is buried inside the transaxle, and is basically always locked.
Something else I found somewhat interesting was that, on at least some of the setups, the front and rear diffs us dissimilar gear ratios. Weird. The rear driveshaft is spun at a different speed relative to the non-existant front drive shaft; and the rear diff is sized to accomodate that. I think they slow down the rear driveshaft, probably to increase mpg and/or reduce vibrations.