Sock calf increases

I can not understand how to make this increase. It is for shaping the calf of a knee sock in the center back of the leg.
What my pattern says is this:

Knit to marker. Insert right needle into stitch just below the stitch you just knitted and knit into that stitch also, m1 (2 stitches increased)

What kind of inc is the first inc ? Do I really knit into the stitch below which is on the right needle? If there is a video here that shows this double inc please send me the link.

Thank you so much!

Sounds like this increase. a KLL, which is a very nice way to increase a stitch and can also be paired with a KRL (also under the Increase videos). The pattern probably gives you a method for the m1 increase so that you increase 2 sts total.