Goflight MCP - A/P CMD versus DISENGAGE - explain the d

When would one use the DISENGAGE instead of just clicking the A/P CMD off? Also I checked the box to have the ARM A/T tied to the A/P CMD and DISENGAGE, but it looks like DISENGAGE leaves the ARM A/T switch UP ( on ) when I use it - atlease when looking at the 2d 737 panel.Any ideas/comments?Thanks,Myles