This is an alternative way to hem knits. Set up serger for a narrow flatlock, or is your manual does not mention it can do a flatlock, most serger can, it is the setting that make it a flatlock stitch.
Basic setting for a flatlock stitch are, needle very low tension, upper looper medium tension, and lower lopper at medium/high. Cutting blade disengaged.
Today I am using my Singer Finishing Touch (14SH654), setting at Needle 0.25, upper lopper 5 (scale 1 to 9), lower looper 7 (scale 1 - 9). Using regular cone serger thread for all. Setting on you machine may be different and with different fabric may have to tweak the settings a bit.
Special note, I have two sergers, and Singer Professional 5, and a Singer Finishing Touch. My Professional 5, will do both a flatlock with raw edges together or one layer folded over the other depending on tweaking the settings.
My Finishing Touch, only wants/can do a flatlock with a folded over layer. Generally I prefer a folded over layer, gives a bit more strength to seam.
Using this method to hem, you might want to setup you machine to raw edges butted, a bit less bulk in the looper side of the seam.
Prepare the fabric, do a double folded hem, and do the flatlock seam on the last folded edge. When sewing/sergering try to get a very small overhang over the edge of fabric will help it lay flatter when done.
After, pull seam open and stretch the fabric to expose the ladder side of stitch, which will be on the exterior of the garment, and the looper side on the interior of the garment.
And there you have a stretching hem on knits, without using a coverhem and a nice looking hem without any fabric seam stretching of fabric or puckering.