The objective of the algorithm is to approximate the curve formula_1 using pixels; in layman's terms every pixel should be approximately the same distance from the center. At each step, the path is extended by choosing the adjacent pixel which satisfies formula_2 but maximizes formula_3. Since the candidate pixels are adjacent, the arithmetic to calculate the latter expression is simplified, requiring only bit shifts and additions. But a simplification can be done in order to understand the bitshift. Keep in mind that a left bitshift of a binary number is the same as multiplying with 2. Ergo, a left bitshift of the radius only produces the diameter which is defined as radius times two.