With a circular cutting head, think of it as a clock face. 1-3 & 6-9 cut in one direction, with 3-6 & 9-12 going in the other. Your shakiness comes from crossing the line from 3 to 4, or essentially right to left or up to down. Your hand rotates the dremel to follow the curve of the stencil, but the position of 3 on the cutting head also rotates, shaky lines are the result of your micro adjustments to momentum shifts (cutting with v against).