How to use the differential adjustment knob on the Juki MO-644D. Not seeing instruction in the manual

Found the manual here on features page http://jukihome.com/products/mo644d.html
Differntial feeding is covered on page 25, and the diagram showing location of the differential feed knob and stitch length knobs inside the cloth plate is on page 2.

So swing open the left cover (cloth plate) to access these controls.

Differential feed means you change the feeding speed of the two feed dogs so the front one feeds at a different speed to the back one. Effectively, this either streches the fabric as you seam, or pushes the fabric together as you seam. So if you want to do a rolled hem "lettuce" edge you are stretching the fabric with the differential feed to achieve this, turn the numbers up. If you are sewing a loose knit and it is "waving" then turn the diff feed down to 0.7 or so to squeeze the fabric back together.

That's basically it! Like anything on an overlocker, test sew, adjust, test sew, adjust until you are getting the right effect.