How could a thick atmosphere keep a tidally locked planet from freezing on its dark side?

A thick atmosphere can prevent a tidally locked planet from freezing on its dark side by facilitating the redistribution of heat from the dayside to the nightside.

On a tidally locked planet, one hemisphere is permanently exposed to stellar radiation, while the other remains in perpetual darkness. A substantial atmosphere acts as a mechanism for heat transport, moving energy across the planet's surface through atmospheric circulation. Furthermore, if the atmosphere contains greenhouse gases—such as carbon dioxide, methane, or water vapor—it can trap infrared radiation emitted by the surface.1

By absorbing this longwave radiation and re-emitting it in all directions, these gases increase the total thermal energy retained by the planet. This process effectively moderates surface temperatures, preventing the nightside from losing all its heat to space and maintaining a thermal equilibrium that might otherwise be impossible in the absence of a significant atmosphere.1