Take hold of a suitable weight dumbbell and sit at the end of a flat bench. Leaning forward, let your lifting arm hang down and wedge your elbow into the meaty flesh above your knee and and inside of your thigh. Your palm faces away from your body in an underhand grip whilst the non-lifting hand is placed on the knee for support and balance. This is the starting position.
Commence the movement by curling the barbell up from the elbow joint and squeezing your biceps. The elbow should work like a hinge with the forearm being the only moving part and the elbow not leaving your knee. Exhale through the movement. Do not use momentum to throw the dumbbells up.
When your biceps are at maximum contraction, squeeze for a second and then reverse the movement by extending the arm to the starting position, inhaling as you do.