Product review (1 stars): Item I received was defective - it broke right away!
I hope it was my poor luck, and hope it's not the case with all of these bump knobs sold under this p/n:
The bolt on the bump knob came out of its hole and the plastic bump knob fell off the very first time I tried using it after putting it on my trimmer.  Thus, the "bumping" feature on the trimmer is forever disabled.  I did not bother getting a new one - figured that I don't want to take chances of getting another bad one.