Cutting on Action or match cuts are one of many editing techniques that keep the audience engaged. It seems almost absurd to think that it would be difficult for someone to finish watching a 90 second video, but it happens.
Match cuts are one of the most basic ideas in editing, and they're especially good for trailers. Simply put, a match cut is when there's an action or motion that is cut off part way through, and then the following shot continues or finishes that motion. It doesn't even have to be in the same direction (though that helps) but this is the basic idea.
Look at this GIF I made from two shots in the teaser I cut for the upcoming game "Pode":
In the first shot, Bulder the rock jumps into the air, and starts to descend, but before Bulder lands, there's a cut and we see Glo landing on a platform in the next shot. Had the first shot finished and/or the second shot started sooner then the two shots would've been self contained moments, but since they're connected by a match cut, it makes them feel like one shot. The match cut has almost "tricked" you into watching more. Cutting before an action resolves is like editing a micro-tease in the trailer.
Think of it this way: If you see a person jump, you want to see them land, right? If you see a rubber band being stretched, you want to see it get released. If you don't see the end result it's like an interrupted sneeze; it throws off your rhythm, and it's unsatisfying. Cutting on action takes advantage of that desire to see a completed action.
Every little motion can be thought of as a small moment of suspense, especially if it's an action that the audience knows the obvious end result of.
- Jumping in the air
- A sneeze coming on
- Winding up a punch
- A baseball bat being swung
- A person about to cross a finish line
There's suspense in all of these scenarios; even if the suspense lasts for a few seconds. This is also important for trailers because oftentimes the shots are very fast, but if you combine the shots using match cuts, then the shots can be even shorter than if they stood alone.
This isn't to say that a trailer should be one uninterrupted series of cutting on action; when to use and when not to use a technique is part of the creative process.
This isn't a trailer, but watch this video by Isaiah Howard who makes AMAZING music videos using the musical.ly app. His videos absolutely blow my mind because of his amazing use of match cuts. He made match cuts the entire aesthetic of his video, and the end result is spectacular.
Match cuts also show intent behind the editing. One way that trailers lose my interest is if I can't detect any intent or thought behind the shot selection and editing. If a trailer seems to be a bunch of random clips that aren't meaningfully put into sequence then I have no idea if it's going to go on for another few seconds or another minute.
So if you're making trailers, consider how match cuts can guide the audience into the next shot or make two shots into one. If shots don't connect visually, the editing can feel choppy, but match cuts can smooth it all out.