Enge,
You might be thinking of a few different operations lumped into one. Here’s what I can think of.
Using a REPLACE edit overwrites and entire clip in the timeline with footage from the viewer. This is a playhead-playhead edit, and the clip to be overwritten in the timeline is determined by the playhead position and track sourc-destination patches. Basically (all in one step), this edit looks down the playhead to the current destination tracks and deletes the clip it finds. It then looks at the viewer and places the frame of video that is in the viewer (at the playhead position, not the in point) into the now empty destination track in the timeline at the timeline playhead position. It then trims out the In and Out points of that single frame so that the hole left by the deleted clip is filled.
Using MARK TO MARKERS (control+A) places an IN point at the closest marker to the left of the playhead, and an OUT point at the closest marker to the right of the playheaad. So place the playhead between markers, press control+A, set an IN point in your viewer, and do an overwrite edit, which will be IN-IN (not playhead to playhead). FCP will automatically fill the marked region in the timeline with the new clip. Remember you can do this my marking and OUT point in the viewer too and backtiming the edit.
I’d say, regardless of whether you’ve placed a lot of beat markers or made a lot of edits on beat that you now want to replace footage between, the second option is best. The first is better if you know that you want part of a clip in the timeline, but don’t care how it edits in or out. But if want a specific in or out point, you can’t get that with the replace edit faster than you can with three point editing.
That’s all I can think of. If there is a hot key that performs and IN-IN replace edit and you figure out what it is, I’d love to know.
Best,
Ben