Mickey-Mousing is the act of synchronizing musical structure and beats to specific on-screen actions. It's called "Mickey Mousing" because the technique was commonly used in old Disney cartoons. You know, when Donald Duck falls off a cliff and you hear a downwards chromatic scale, capped with an orchestra hit when the unfortunate fowl hits the ground. This is also called "scoring to visual rhythm".
This technique works quite well for animation, but can feel a little out-of-place in live action films, where is can feel particularly comedic, dopey, or jarring. So when it IS done in lave action films, it's usually played down a lot, or played to comedic effect.
The use of Mickey-Mousing in a film is going to depend on the genre of the film and the vision of the director.
In a lighthearted film like The Screenwriters, the aesthetic and flavor of this film can accommodate some more mickey mousing than a more serious drama, like Polycarp, where this approach feels very inappropriate and irreverent.
HIT POINTS AND SHIFT POINTS
A hit point is a spot when a beat or hit in the music is intended to sync with a specific frame or act in the video to emphasize that precise moment. Like a Brass stab syncing with a punch or a door slam, or a glockenspiel strike emphasizing a wink for comedic effect.
A shift point (or turning/pivot point) is a place in a scene where the emotional task of the music in the cue shifts from one thing to another. Shift points are not as obvious as hit points, and may be completely seamless and unnoticeable, evolving over the space of 30 frames or 30 seconds: a shift point is a fulcrum around which the direction of the drama in a scene makes a definite change.
Watch a few old Disney cartoons to see the technique in action as it existed in the early days. For a more developed use of the technique, follow this up by watching Bruce Broughton's masterful score to "The Rescuers Down Under", which includes some of my all-time favorite mickey-mousing. Also consider watching John Debney's "The Emperor's New Groove" for a fantastically funny use of the Mickey-Mousing technique.
Watch three scenes from favorite films of yours, and see if you can identify the hit points and shift points in in those cues.