It's like that feeling when you're listening to a song and you can tell that its tempo is growing steadily faster. Maybe subtly at first, but your brain picks up on the pattern.
When it's unmistakable that the song is getting faster, you next notice that it's continuing to accelerate.
Then you become aware that it's heading inevitably to a point where the tempo will outpace what the musicians can possibly be expected to play.
The song hasn't ended yet, and you don't know when it will, but you see where its trajectory is taking it, and you know that it's about to become unsustainable.