It sounds like you're asking about torque converter "lock up". This is when the input shaft of the transmission is "locked" to the output shaft of the engine. Under normal circumstances, there is a slight amount of slip in the torque converter between the engine and transmission. To improve fuel economy, modern transmissions can lock the torque converter to prevent slippage. In most transmissions, this is indicated as "overdrive". Engaging tow/haul mode, turning off overdrive, or moving the shift lever from D(with circle) to D will prevent lockup.
