[plt-scheme] Clarification about control key behavior

Question about control keys on Windows XP Pro with DrScheme v371.

c:m and c:r split and collapse editor windows as expected

I would like to be able to use the c:x o and c:x p to navigate between
the split windows, so I went to preferences and unchecked "Enable
keybindings in menus".

The previous and next navigation works correctly, but now the c:m and
c:r no longer work.

How does this work? This is not what I expected.