The inner plunger has a c-clip on it that clicks past grooves in the outer housing. This allows it to move inward, but not move back out again. Just be sure to install the inner plunger last, after you have installed the outer sleeve; then insert the spring and tighten the cap.
Yes, the spring and cap are a real bear to install; and there is a lot of pressure on the spring; feels like 50 pounds when you are trying to hold it down and turn the cap. Make a tool of some sort out of a bolt, so you can stick the bolt head in the tensioner cap and wedge it against something while you turn it.

DG