Mike Rivers - who authored some of the Mackie manuals, and also published a "Mackie Compact Mixer Reference Guide" - has elaborated on this "best way to insert" topic, on various occasions in r.a.p. newsgroup threads.
For instance, here's his explanation from a thread of 2006:
> The mixer has no per-channel line-level outputs except for the INSERT
> jacks. But plugging a 1/4 mono (e.g. unbalanced) plug into the INSERT
> jack cuts out the signal. If I partially insert the mono 1/4 plug into
> the INSERT jack I can get both a signal to the recorder and the signal
> through the board. But that is an awful setup - any slight bump of the
> plugs and we either lose the recorder signal or the live signal. Ugh.
It indeed looks suspicious, but you'd be surprised at how many people
work this way without a problem. You just have to be careful. However .
> Any sugggestions on how to configure this? Will a 1/4 inch
> stere-to-mono adapter do the trick? I assume it just needs to short
> the right/left channels of the stereo plug to prevent the board from
> losing the signal?
That's exactly what you need to do, however, sticking a stereo-to-mono
adapter on to the plug adds another couple of inches of lever to what's
straining the jack. If you don't properly support support the cable to
take the weight off the jack, you'll have a jack failure sooner than
What you really need to do (you can't buy this off the shelf) is build
some cables that have a TS plug on one end and a TRS plug with the tip
and ring terminals connected together on the other end. Buy half as
many 1/4" TS cables as you need, but twice as long as you need. Get as
many TRS plugs as you need. Cut the cables in half and solder the TRS
plugs on to the cut ends.
After I encountered my first intermittent signal failure, I stopped using the half-insert method for all critical recordings [<= but aren't they all? :-) ]