There is an internal clutch that locks the converter when engaged by the torque converter clutch solenoid. Fluid pressure it routed through the input shaft into the converter clutch, which engages the clutch.
If it is non electronic transmission it uses a pressure switch inside the transmission to engage the solenoid (gm calls it a tcc solenoid) at a given speed and current gear (gear is usually 3rd and overdrive, if electronic, the trans computer controls lockup using the solenoid. When it engages it depends on make/model of transmission and software programming.
