Where did the phrase "drop the hammer" come from? It's what you do when you start to go balls to the wall.
I've only heard it rowing.
I've only ever seen it as described in definition-of.com: "Bringing a pending act to fruition. Usually connotes an act which will have serious consequences" and always as a one-time act which will bring all force to bear.
Also, like loufedalis' answer given in Yahoo! Answers, I believe the origin to be military and related to the hammer of a cocked pistol or rifle. When you pull the trigger and drop the hammer that certainly fits the poetic description of someone waiting for the right moment to strike with all available force.
From Wikipedia...in Reference to repairing the clock "Big Ben" in London...."3–4 June 1941: The clock stopped from 10:13 p.m. until 10:13 the following morning, after a workman repairing air raid damage to the clock face dropped a hammer into the works." Or another saying it resembles..."Put a spanner in the works"???
This must be a reference to Thor's hammer! Thor was a major Norse god who wielded a fearsome hammer, which was his weapon of choice. He was the god of thunder, and his mighty hammer called "Mjollnir" was forged by dwarves. When Thor did battle, he dropped the hammer on his foes. When people heard thunderstorms they knew it was the sound of his powerful hammer crashing down on his enemies.
So to "drop the hammer" is to go all-out, or give it your all (balls to the walls).