This increase can be confusing at first, but not bad once you get the hang of it. I use it because it is a good tight increase and it does not distract from the pretty designs in the body of the sweater. On some of my older patterns I called the stitch in between the two makers the seam stitch. Now I call it the raglan stitch because that is less confusing.
Use the RH needle to pick up the st below the raglan (seam) st on the LH needle.
Place it on the LH needle, then knit this new st.
Sl m, knit into the back of the raglan (seam) st, sl m.
With the LH needle pick up the left side of the same stitch in the row below the raglan (seam) st.
Knit into the back of this stitch.
An increase of 2 stitches.
How it looks used as a raglan increase.