WEAVING IN YARN ENDS WITH DIAGONAL DARNING
This method is close to invisible, unlikely to come loose, and since you weave on the bias (diagonal), the knit fabric still stretches lengthwise and widthwise.
Always weave in yarn ends on the wrong side of the work if there is one, although this method is hard to see even on the side where you weave in. I weave the yarn end diagonally for about 1-1.5”, then turn the work and weave it next to the original path for about 1”, then turn the work again and work a 3rd line next to the 2nd. The photo above shows a contrasting color yarn woven in, with the needle inserted for the 3rd pass. For wool, mohair, or especially grabby yarns, 2 passes may be enough.