It looks like you’ve got the spring-loaded lock collar in the lock position. Press down on the collar against its spring and the main coil spring inside the tube should snap the core cable back to the retracted position.

To set the cable for normal operation, press down on the collar and as you hold it down, rotate the collar to lock it in the down position.

For long bulb exposures, rotate the collar the other way and it will spring upward to the automatic-locking position. Then when you depress the plunger it will stay depressed without having to hold it. When you want to terminate the exposure, press down on the collar and the core cable will retract.