Confused about this pattern

I’m making these mittens that are technically fingerless, but have the closed flap that you fold over your fingers. I’m stuck on the closed mitten part of it.

The pattern reads…

Mitten Top
Setup round: BO 7 sts, k until you have 16 sts on the right needle, BO rest of round. Break yarn, leaving a 12 inch tail.
Rejoin yarn.
With RS facing and using backwards loop cast on method, CO 7[9] sts, p across 16[18] mitt stitches, CO 7[9] sts. 30[36] sts.

Can anybody clarify the “rejoin yarn” here? I just bound off the appropriate stitches, and the 16 stitches that are still on the needles are located on the front side of the mitt (the back of your hand), but the end of the round where the yarn tail ends up is on the opposite side (the palm of your hand). So if I just rejoin the end of my yarn ball to that yarn tail and do my cast on and continue knitting, there’s a long piece of yarn that connects one side of the mitt to the other, which is obviously not right. Does that make sense?

Also, what is the point of leaving a 12 inch yarn tail? If you’re just re-joining it, then why does it have to be 12 inches? Or, by “rejoin yarn” does the designer just mean that you pick up the end of your yarn ball and cast on with that, and leave the 12 inch tail hanging for later? I’m so confused! :??

To ‘rejoin yarn’ you’re going to cut the yarn and use the new end to begin knitting in a different spot. Just leave a long enough end to weave in later. I’d leave the 12 inches, you might need it to do a bit a seaming later; cast on your sts, purl the remaining ones and cast on the other sts. You can use the knit cast on, it doesn’t have to be the loop.

Thanks! As always, you are awesome