Spindle Sander Design Question

[calandrod]

I have a B4 State oscillating spindle sander. Can anyone tell me the purpose of these sections highlighted in red? They rotate as one unit independent of the rest of the set up. Is there a reason for them?

 

[RTM]

Manual here, might help you. Looks like its part of the "positive lok" system on page 7

http://www.vintagemachinery.org/pubs/798/2378.pdf