What PA said, I use it all the time on figures and buildings. Spray undercoat black then use a large brush slightly wetter than you would use for drybrushing to go over the whole thing with white. The aim is to hit everything but the recessed areas and pick out all the detail.
For me it serves two purposes, firstly it makes it easier to see the detail on the figures. Secondly, especially if you use slightly thinned paints, it gives an initial shade and highlight automatically with no effort.
It's not a technique I've ever used on 28mm or larger figures, for those I'd usually go with white undercoat and full shade and highlight. But for 15mm or under it works very nicely.
It's generally something you use for speed painting miniatures and with textures like fur, chainmail and stone. I mostly use it for getting the second colour on when painting the sand on bases. If you imagine normal painting at one end of a spectrum and drybrushing at the other then overbrushing is in the middle ground and all the increments in between. In practice there's not much in the difference between over brushing and drybrushing but overbrushing tents not to have a chalky look.