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.