The problem is that oftentimes people don’t actually twist the wires together, they wrap one around the other. This is most common in cases where the gauges are different or joined solid core to stranded, because one wire is more flexible than the other. Basically, you want to see the two wires spiraling together, like a DNA strand from a high school biology book, and not one wire spiraling around the other untwisted wire, like one wire is the red stripe on a candy cane and the other is the white core.
If you end up with the latter, the wire-nut is only gripping the outer wire and the inner core can slip free. So while the technically best scenario is to properly twist the wires together and then cap with a wire-nut, the second best option is to put both wires straight into the nut and use that to twist them together as it will have at least some purchase on both wires that way. And using the straight into wire-nut makes the same connection reliably, whereas twisting first may instead wrap one around the other, which is a distant third place in terms of connection quality.
Jackoffalltrades89 t1_jd357dj wrote
Reply to Why should wires not be twisted before putting in wire nut? by dhekurbaba
The problem is that oftentimes people don’t actually twist the wires together, they wrap one around the other. This is most common in cases where the gauges are different or joined solid core to stranded, because one wire is more flexible than the other. Basically, you want to see the two wires spiraling together, like a DNA strand from a high school biology book, and not one wire spiraling around the other untwisted wire, like one wire is the red stripe on a candy cane and the other is the white core.
If you end up with the latter, the wire-nut is only gripping the outer wire and the inner core can slip free. So while the technically best scenario is to properly twist the wires together and then cap with a wire-nut, the second best option is to put both wires straight into the nut and use that to twist them together as it will have at least some purchase on both wires that way. And using the straight into wire-nut makes the same connection reliably, whereas twisting first may instead wrap one around the other, which is a distant third place in terms of connection quality.