“The gameplay beats show the main events and activities that the player will undertake and how these are spaced out and timed in your level. This also covers the narrative beats; the storytelling within your level, as ideally they will be intertwined with gameplay.”
Its important to understand what will be the rhythm imposed on a stage and how each section is spaced as the player progress. Normally the gameplay beats are presented using a line or a bar graph to understand how the narrative and the game is moving along Like the following one presented on World of Level Design
Another possibility is to do a pacing analysis focused on how each mechanic is paced around on a game, something similar with what was presented by Filip Coulianos on his analysis on Batman Arkham Asylum using a Flow Chart
Both ways will help to plan where a new mechanic should introduced to the player or where the boss fights happens inside the stage. Another possibility is to analysis where we should be placing a hard-gate, an area where the player can no longer came back after he passes it,