WRC cars drift because they are usually slightly front heavy (engine, tranny, front diff). They do compensate this by putting the driver and co driver further back, but it doesn't help completely. So on loose surfaces (snow and gravel) they oversteer a bit (drift, powerslide, whatever) to compensate for the front heavyness and prevent the understeering from happening. That's all. Go play RBR and go through a gravel hairpin the normal, racing way and then go through it while oversteering. What's better?
EDIT: Or use the RB4 on FE RallyX track (the long one). There's a nice hairpin there.