Pinned reels let you have a recording, your highlights, or another reel in the central panel, at the same time as having a reel visible on the right side of your screen. You can drag items to the pinned reel, and drag to rearrange, just as you can with a reel in your central panel.

A pinned reel will show up on the right side of the screen when you "Add to reel" or "Share as reel", or by explicitly pinning a reel.

This is a reel that is already open in your central panel; click this icon to pin that reel to the right of your screen.

And this is when looking at the listing of your reels in the left sidebar. Clicking "Pin reel" will not impact whatever you have visible in your central panel.

When you drag items from one reel to another, the item is copied (and not moved/deleted from the original). It also brings with it any strikethroughs that were created in the first reel.

Note that in this configuration, you can drag from the left sidebar into either the reel in the central panel or to the pinned reel, and you can drag in either direction between the two visible reels.