John Deere 39 mower question

[JimPS]

My neighbor got a JD model 39 sickle mower, but we haven't gotten it hooked up and in operation yet. There is a part of the machine that I'm really curious about, as to when and how it works.

There is a large spring connected to a shackle, and that shackle appears to slide up and down an arm that's about a foot long. Under what circumstances does the shackle slide on the arm?

Thanks,
Jim

 

[Tx Jim]

Part encircled in red in your photo is long to allow lift clevis to slide the length of part if cutter bar hits an obstruction & cutter bar "breaks back" from spring loaded catch encircled in green.

 

[JimPS]

Okay, makes sense now. Once we see it in action, we might experience it although the fields are pretty clean. I suppose if you cut too fast it might come back too. Thanks a lot for the info! Jim