Can someone with please explain how the trigger spring tool is used?

I have the trigger spring tool but from my unskilled take on it the twist tie was much simpler use. Compressing the spring into that tool was a job. I guess I just don't understand how it is set up and used for the rebuild.

I used a soft covered twist tie to keep the trigger spring in place and also to hold the hammer forward while replacing the hammer spring/plunger.
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