For the side with no seed, simply separate the two halves into two parts. For the side with the seed, repeat the same action you did before. Firmly grip the two sides and rotate your hands in opposite directions. One portion will slip off the seed.
Now, you are left with one quarter with the seed. Do not attempt to "hack" at the seed with your knife. This is too dangerous! Instead, place the avocado quarter on the board, with the skin side perpendicular to the board. Secure the avocado to the board with your bear-claw grip. Make sure none of your fingers are near the knife as you push the seed out (away from your body) using the blade of your knife, moving parallel to the board. It should pop right off!