What makes the yardstick test a direct measurement of reaction time?

The term yardstick refers to what is a normal response.  It really has nothing to do with a yardstick and can be used in many situations.  A yardstick is a measurement that has been tested over and over and than that measurement becomes the standard.
Say the yardstick for a reaction time is 5 seconds
Meaning that is what a normal healthy person can do it in.
Joan has the reaction in 4.8 second.  Well she is on the the bell curve and did better than the yardstick test measure.

Yardsticks have numerical markings for length. In the yardstick test we can directly compare the distance dropped ( in numerical marking) with the reaction time. Hope that helps!