The Pancake Timeline is a video editing technique that I first discovered thanks to editor Vashi Nedomansky, who also relates that David Fincher's editors, Kirk Baxtor and Angus Wall, also make use of it.
So it has a strong pedigree behind it!
It's a simple but visually intuitive approach to editing in which you stack two open timelines, one above the other, so that you can move selected clips from the source string out (top timeline) to the edited timeline (below).
This is similar to opening a string out in the source monitor but it's all there in front of you.
Pancake Timeline - The Shortcut Version
Editor Vashi Nedomansky shared a great tip for speeding up your pancake timeline editing in this detailed article on the editing of 6K feature film 6 Below, back in 2017.
For 6 BELOW I developed a custom keyboard shortcut pattern that allowed me to effortlessly plow through hours of footage and build my sequences. This is how I spend the bulk of my day so I made sure to make it as simple as possible:
1) Open sequence with select footage into Source panel (keyboard shortcut)
2) Open sequence in timeline from Source panel (keyboard shortcut)
3) Enable Timeline preference “Set focus on the timeline for edits”
4) Choose In and Out for the shot you need from Source timeline
5) Insert or Overwrite the shot into the main timeline (keyboard shortcut)
3 keystrokes to choose shot and non-destructively move my range of footage from one sequence to the other. No dragging full clips and trimming later after the fact. Just the exact chunks needed at that precise moment.
Plus I can do this without taking my eyes off the monitor as my fingers just progress down the keyboard in order.
— Vashi Nedomansky, Editor
I talk more about editing feature films in Adobe Premiere Pro here.