Small trenches are run into each end of the solid shaft.  Steel wire is bent by hand to form springs, which are inserted into these trenches.

The springs at the bottom of the shaft secures the runner when the umbrella is closed and the spring at the top secures the runner when the umbrella is open. 

The springs have to be bent and fitted by hand as each natural wood shaft is a different size and the shaft can be ruined if the springs are not fitted correctly and catch on either side of the trench.