Do you know how to use the "M" key on the Brother KH910 please?

I have having trouble getting the patterning right when knitting a round neck sweater. First half knits beautifully but when I take the garment off the machine the second half does not match the first. I have followed the manual exactly but cannot get it to match. I have marked in the L window on the mylar sheet where the pattern starts for the first half. When I come to knit the second half the mylar sheet is at the exact position shown by the marker. Someone suggested using the M key which I have never used. Do you know how this works. Thank you in advance.