Question about a motor "part '

[ViperTech]

I hope I am posting this in the right place!
My son bought me a Firepower 370 sport 2826-14 motor in exchange for my 2826-6 2200KV Turnigy motor.
He wants the high KV for a FT-22 he is experimenting with. I am wondering about the parts that came with it, it has a prop drive adapter but it came with a round collar that has a grub screw in it. What is that for? I have no instruction and am clueless as to it's purpose. Any thoughts?

View attachment 22838

 

[aiidanwings]

If you revered the shaft (stuck it out the other end of the motor) that collar would be used to hold it in place. Because there isn't a groove on the shaft for reversal.