This feature works when you hold shift then click a link, that link opens in a pop-up above your current website without requiring a full new tab being opened. That's super, super convenient especially when you just want to don't want to lose your place on a page but do want to explore the link. Then if you do want to open the link in a full new tab, you just hit a button to do so and bam, now it's a new tab.
No joke, this is the feature that originally kept me on Arc for so long, then when I realized Zen had it, I switched to that, then eventually to Firefox for other reasons, but man I miss the "glance" link popups. It is the only reason I still sometimes think about switching back to Zen.