Rendering individual layers (NOT Final renders)

Hello, I see someone doing this on a video and I forgot to save it and can't find it because it just happened to be while they were doing something else and not the topic of the video.

How do I render individual clips so that they no longer need to be RAM previewed, to make editing faster when a specific layer is isn't going to be edited anymore?

I seen this guy doing it and it showed some little construction cone or something next to the layer so that he didn't have to RAM preview it so he could edit faster without having to RAM render the finished layers over and over again taking so much more time.

Is it the Pre Render?  And if you use that, does it use that rendered file for the final export or does it use the original, or do we have to replace with original before export?

He did a quick show how to do it and I was half asleep and somehow though I would just go back to the video.....that I didn't bookmark =/.   I can't find it in any of the drop down options.