It is in the syllabus.
sgn(b) is the sign of b. So if b > 0, sgn(b)=+1, if b < 0, sgn(b)=-1. If b=0, sgn(b)=0, but then there's no need for the formula.
Using this formula is a much faster method and so as far as risk is concerned, there's much less chance of making a mistake if you...