Why does your transmission lock up when put in gear?

Transmissions don't typically "lock up" when put in gear. Most modern vehicles use some form of an automatic transmission. Automatic transmissions use hydraulic pressure and bands or clutches to engage and disengage different combinations of gears.

However, some vehicles equipped with automatic transmissions may have a feature called a torque converter clutch. The torque converter clutch acts as a direct mechanical connection between the engine and the transmission, locking the two together and increasing efficiency at higher speeds. When the torque converter clutch engages, it reduces slipping and heat buildup within the transmission and improves fuel economy.