This rule is part of the Axis & Allies Global 1940 2E, House Rules Expansion.
The Escort unit is a sea unit, that has multiple tasks. The primary task is to protect Transports as part of a Convoy.
Naval Base Protection
Before each combat round, all cruisers and battleships defending in a sea zone adjacent to a friendly operational naval base will receive 1@1 each for a chance to become protected by naval base defense systems.
For each 1 rolled, the defender may choose to protect up to that many ships by removing them from the battle board, and placing them back in the sea zone where they were defending. Also, if 1 or more air units are scrambled into a sea zone adjacent to a friendly naval base, the protection roll for each defending cruiser and/or battleship becomes @2 or less.
If the naval base protecting ships has changed hands during the same turn it was used to protect ships, the protected ships are destroyed and immediately removed from the board.
This sounds like an interesting idea, but why just BBs and cruisers?