Refers to the final act or climactic scene of a play, story, or event. Often used figuratively for the last stage of any process.
The negotiations have finally reached their final stage.
このドラマの大詰めは本当に感動的だ。
The finale of this drama is truly moving.
The negotiations have finally entered their final stage.
The story is approaching its climax.
A dramatic reversal occurred in the final stage of this game.
大詰め refers broadly to the climactic final stage of any process, whereas ラストスパート focuses on the burst of effort or speed at that stage.