Clip the line and the spool at the same time.
That's what I do, and the line unravels.
I was taught a method to clip off by my previous instructor that will not unravel. Basically:
1. Tie a 3/4 foot long loop in the end of the string
2. Tie a small knot at the top of this big loop.
3. Tie a small knot at the side of this big loop.
4. Fit loop into metal ring of SMB and over the spool. This secures it to the metal ring on the SMB.
5. Pull out small knot at top of big loop, and pull line through the hole in the side of the spool. (#3 serves as a marker of which hole to use).
6. Clip double ender to loop and wrap string around itself and it's gate, then clip off to the metal ring (not the spool) on the SMB.
Using this method, it has never comes loose. I used to clip the SMB on my butt D-ring and it has never come loose using this method.
Downside is using this method requires a longer time and more fumbling to undo when you want to shoot the bag.
I was wondering if there was a better/simpler/faster way to do it, so it does not come off, but is fast to deploy.
There is a lot of string on my spool (30m), I don't know if that has anything to do with it coming loose all the time if not secured with the above method.