Applying either brake causes weight to shift onto the front wheel. The rearward force of the ground on the tire creates a rotational moment. For a given amount of braking, the amount of rotation is the same regardless of which brake is being used.
The limiting point of braking comes at one of two points: either when the tire or brakes reaches the limit of traction, or when 100% of the weight has been transferred to the front wheel and the bike becomes unstable. Typically, the rear brake is limited at the first point, and the front brake is limited at the second point. If the front brake is capable of lifting the rear wheel -- achieving 100% weight transfer -- then that is maximum braking for that bike. If the rear wheel is off the ground, it is not contributing to the braking.
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