A&B are those metal lines with WIFI under it. For instance, spark the last two on both A and B and you've filled in 3x3. (Most right one is worth one, the one left of it 2, the one left of that 4, one more left and you get 8, and the most left one is worth 16. So spark both 1 and 2 and you get 1+2=3. Largest number you can make this way is 961 as 1+2+4+8+16 = 31, 31*31 = 961.)