That would be because multiplying any number by a number with an absolute value of 1 gives you a number with the same absolute value (8 × 1 = 8, 56 × -1 = -56). Multiplying any number by a negative gives you the opposite sign of the number being multiplied by the negative (- × - = +, + × - = -). Therefore, multiplying a number by -1 gives you both an opposite sign and a number with the same absolute value that you started with.
