Whether or not the challenger literally stood on one foot, and whether or not Hillel had a regula (measuring rod) in his hand as Shammai did, Shammai’s regula was used to reject the challenger, whereas Hillel’s regula (= מדה = rule) was used to bring him to the Torah.

Raphael Jospe, “Hillel’s Rule,” The Jewish Quarterly Review, New Series, Vol. 81, No. 1/2 (July-October, 1990), 56.