Initiate the drift with a hard stab on the brakes, then leaning hard into the turn. As I said, do your braking before the turn to set your speed. If you watch cars drift, they stab the brakes to initiate the drift, then apply throttle to control the drift. On an MTB you can't stab the throttle to control the drift, but you can release the rear brake after intitiation and let the rear tire rotate freely (which incidentally is easier to control than a locked tire anyway) and shift your body to control the drift, or just let the bike drift around the turn. It takes a lot of practice and knowing which line to take in a turn. You want to aim to get as close to the apex as possible so that the bike can drift to the outside a bit. The type of drifting your doing isn't a true drift in the sense of cars or bikes. You're doing a pendulum turn (rally racing) which rotates the rear of a car quickly for sharp turns. Even then, the time the brake is applied is very short as the car is still controlled by the powered wheels (the front wheels). Drifting is a way to maintain a high rate of speed through a turn. Your locked wheel will actually slow the bike through the turn because it isn't rotating and resisting forward momentum.