Up translation occurs when the A-side is driving the signals to the B-side. Remember that the higher voltage supply should still always be connected to the B-side. For a low signal driven from the A-side, current flows from an external driver such as an NMOS transistor. The pass transistor conducts and the B-side is pulled low also. For a high signal driven from the A-side, the pass transistor will switch off and the B-side will be pulled high via its external pull-up resistor.