I've learned that he did two things: first, he shortened his crank arms to 170 or even to 168 (depending on the course and most often just for TT's) and trained using 53/15-16 gearing a lot for longer periods. This "higher cadence" over long periods is said to help quiet hip movement which can give a more "round" pedal stroke and therefor a better energy transfer. The story about picking strawberries is true. Strong backs are certainly an asset. Merckx himself said that the crash he had where he hurt his back, almost cost him his career. In his words: "I was never the same after that..."