A beat sheet is not a summary. Each line states the turn: what a character wants going into the beat, what stops them, and what is different coming out. A list of events that does not name what changed is a running order, and a running order cannot be diagnosed because nothing in it can be wrong.
The reason to build one before writing is economic. A beat is a sentence and costs nothing to throw away; a draft costs a day and you will find yourself defending it. Every writer who has argued for keeping a scene they knew was dead was arguing for the hours, not the scene.
In generative production the beat sheet does a second job: it is the document the shot list is derived from, so it decides how many shots exist and therefore what the piece costs. Changing a beat is free. Changing it after four hundred renders is not.
What is the difference between a beat sheet and a synopsis?
A synopsis says what happens. A beat sheet says what changes at each step — the want, the obstacle, and the state at the end of the beat. Only the second can be tested for structural problems.
How many beats should a short film have?
Fewer than most drafts contain. A ninety-second piece usually supports six to nine real beats; anything more and the beats are events rather than turns, which is what makes a short feel busy and empty at once.