actually, the torque converter is always going if the engine is running just like a car. The clutch valve dumps pressure and releases a clutch-pack that is inbetween the torque converter and the transmission. The direct drive is engaged by the lever to the left of the steering column and via hyd pressure and a cylinder engages a clutch locking the torque converter so there is no slippage and is therefore more efficient.
Pretty good transmission systems and you'll love once you get to use it but there are some problems. Plugged filters, warped clutch packs, worn clutches, bad seals (lets hyd pressure leak off), bad pump ect.