Complex sentence: Dynamo took its name from the dynamo room that provided electricity in the naval headquarters below Dover Castle, where Vice Admiral Bertram Ramsay planned the operation and briefed Churchill as it was under way.
Split into simpler sentences: Dynamo took its name from the dynamo room that provided electricity in the naval headquarters below Dover Castle. It was in this room that British Vice Admiral Bertram Ramsay planned the operation and briefed Churchill as it was under way.