constraint equation coefficient 1 and -1

https://www.youtube.com/watch?v=soYwaW54WRg&list=PLMtv9Af75s6I6LMsDX3Defn4XBL9I7xZo

In this example at time 9:11

Constraint coefficient between reference point and part is  -1 and  1. Why ? 

They move same direction, why not 1 and 1, 

Thank you.