Yes, it’s this:
For those not familiar with a torque converter, picture two fan blades facing each other with both submerged in a thick liquid. If you spin one fan blade, you’ll spin the liquid around with it and the other fan blade will get spun by the liquid. But it’s not a solid connection. If you stop the 2nd fan, then the first fan can still spin, you just end up sloshing the liquid around.
This is how the torque converter allows the wheels to stop without the engine also stopping.
Back in the old days, automatic cars were always less efficient than manual cars because of the torque converter. When you stop, the torque converter’s lack of a solid connection allows the engine to rotate with the wheels stopped, but when you are going down the road the engine still isn’t locked to the wheels, so you waste energy sloshing around the fluid in the torque converter.
Modern automatic transmissions use what is called a lock-up torque converter. When you stop, this functions the same way as an old fashioned torque converter, and you just slosh fluid around inside the torque converter while the vehicle isn’t moving. But once you get going down the road, the two “fan blades” lock together to form a solid connection, like what you get in a manual car. This allows the automatic transmission to be just as efficient as a manual transmission.