Mitten thumb question

I’m just finishing up the thumb gusset on a mitten, and the directions say to move the thumb stitches onto a holder or scrap yarn, then CO 1 in gap. What the heck does that last part mean? I’ve done thumbs before and I’ve never had to cast on additional stitches.

The cast on st helps prevent a gap where the thumb connects to the main part of the mitten. Work up to the gap, cast on one st. You can use a backwards loop for this cast on or you can knit cast on a st and then continue working around the mitten.

I have been making mittens and when I place my thumb gusset stitches on waste yarn I finish the round and on the next round I cast on 2 stitches by using a method I learned on You Tube, Tillybuddy’s method. It is a stretchy cast on and it stretches but closes up the gap much better than one stitch does… I knit the mittens using the magic loop method. I don’t have to cut the yarn to rejoin it after the casting on until I close the top of the mitten with Kitchener stitch.

Please, oh please, could you post a link? I’ve not been able to find anything by Tillybuddy on youtube, I’m not searching effectively I guess and need some help. Thanks!