What does "Confirm & cut away red" do?

Confirming does two things:

1. The recording counts as checked — from now on videos can use its green parts.
2. The view switches to the clean cut: only the green clips, side by side.

In the clean cut

• Play clean cut plays all green clips back to back, skipping the red parts.
• Click a clip to play just that one.
• Next recording to check takes you to the next unchecked recording.
• Edit marks brings back the full bar if you want to change something.

Your original file isn't changed. Red parts are simply never used.