Understeer or oversteer happens because you are driving too fast for taking any given curve. Then the traction wheels lose traction and start sliding.
If your car is FWD, your car normally understeers when losing traction. If your car is RWD, it normally oversteers when losing traction.
Actually, the brake balance can help prevent or induce oversteer, in a RWD if the balance is 60% rear wheels, 40% front wheels, then you will probably oversteer by taking a curve very fast and then slamming the brakes.
'Designing the car to understeer' seems to me just changing the break balance so the front wheels brake more than the rear wheels.