Honda Acura NSX 1991 Service Manual page 427

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In in 2nd, 3rd and 4th, pressurized fluid is drained from the back of the torque converter through an oil passage,
causing the lock-up piston to be held against the torque converter cover. As this takes place, the mainshaft rotates at
the same speed as the engine crankshaft. Together with hydraulic control, the control unit optimizes the timing of the
lock-up system. Under certain conditions, the lock-up operation is applied during deceleration, in 3rd and 4th speed.
The lock-up shift valve body controls the range of lock-up according to lock-up control solenoid valves A and B, and
throttle valve B. When lock-up control solenoid valves A and B activate, modulator pressure changes. Lock-up control
solenoid valves A and B are mounted on the torque converter housing, and are controlled by the A/T control unit.