I don't think Antiramie or Pascal have it quite right. Oversteer is a matter of losing lateral traction in the rear tires, while understeer is losing lateral traction in the front tires. Any negative accelaration while turning will transfer weight off the rear tires, which in turn reduces the amount of traction. This will only result in oversteer if the front tires maintain traction. If both sets of tires lose traction, then you get the fun of the 4 wheel drift!
