After-boil is caused by water-based coolant that is at a temperature near its boiling point in an engine that has been stressed by running hard and then shut down. Heat stored in the cylinder head continues to dissipate into the coolant in the head cooling jacket. Boiling and water vapor occur when the coolant temperature exceeds the boiling point for the pressure of the system.
After-boil can be a cause of a mysterious loss of coolant from a non-leaking cooling system that has a pressurized expansion tank. Water vapor from the after-boiling displaces liquid coolant in the cylinder head, forcing the pressure high enough to open the pressure relief valve at the cap on the expansion tank. Liquid coolant is pushed out of the expansion tank and onto the ground.
When using Evans Waterless Engine Coolant there is no after-boil, because the coolant in the cylinder head is much colder than its boiling point and acts as a heat sink with the capability to absorb all residual cylinder head heat without boiling.