This is something that I've always wondered. I'm no mechanic (web developer, actually) with very basic knowledge of vehicle mechanics so I can only guess at something like brake pipe pressure being involved, but that's about as far as I could guess. I can't find any article on the subject either, though I admit it's quite a niche question.
Case in point; I'm leaving the motorway via a downhill off-ramp and come to a stop in the queue using the foot brake (still downhill) - I shift into neutral, apply the handbrake and release the foot brake. As I release the foot brake the rear of the car sinks, as though the suspension was being elevated by the foot brake, though I've no idea how this is possible.
I apologise in advanced if this is rudimentary to those versed in vehicle mechanics and may be obvious. I've come to the assumption that it's normal behaviour and not an issue, but I can't remember whether or not this occurred with my previous two cars.
If it's relevant, here's my vehicle information:
Citroen Xsara Picasso Exclusive, 2006 (06), 1.6 HDI (Diesel) - 92HP, Manual Transmission