If you need to roll a seven you need to roll a seven - so you can't pass, and thus fail the save. Shooting rolls to hit have a specific exception to this built in - you always hit on a 6 - but nothing else in the rules does so far as I remember.
Incidentally, I'm pretty sure a rake is only a -1 save, not a -2. I don't have my book on me to quote page numbers, though.
I'm pretty sure it's just one may not fire marker, so one turn's break in firing unless you're a Brit or American and can pass that end of turn command check. I'm not sure there *is* a crystal clear rule for this in the book, but it seems the most sensible reading of the rule to me.