|[ETA - I've just found out that the following tip is a common everyone else knows but me. Sorry about that.:-) I did check the tips first and couldn't find anything so I thought it worth sharing. Unfortunately I do not appear to have the option to delete it. I must ask Deepika if a delete box could be an added feature.]
I learned about this today and thought it was too good not to share. Since most machines have a single tension release handle or button this is not usually a problem but for some machines like the Brother 2340CV which has multiple buttons, it could be useful.
At the end of your coverstitching, if you manually do one stitch backwards, leaving your needles at their highest point, then lift pressure foot, this will release all the thread tensions. No, it doesn't pull the needle threads through to the back, sorry.
The mechanic at my local store said this should work on all coverstitch machines. I've tried it on my Brother CS machine and it certainly works. There is a downside though in that some stitches tend to unravel when you pull the fabric away, even if you're careful.
While generally, I might use this method as a last resort, it may be useful on cuffs and hems where stitches are overlapped by just allowing more of an overlap to allow for the lost stitches. You would have to pull the needle threads through to the back too.
I think I'll probably stick to Belinda's method for releasing tensions but it's always good to have a backup if all else fails.