The gimmick: In our digital era, split screen scenes (displaying two or more scenes simultaneously on a partitioned screen) don't seem like that big of a deal. But the technique was revolutionary when it first came out and was much more difficult to achieve. Filmmakers had to run dual projectors — sometimes even more — synced perfectly for the film to work.
The result: The 1968 flick The Thomas Crown Affair popularized the technique to show multiple parts of the movie's heist simultaneously. The technique works especially well for the caper movie and modern counterparts like Ocean's 11 owe a debt of gratitude to Thomas Crown.