For the most part a good acoustic seal is dependent upon the part of the mold that goes into and past the first bend of the ear canal, so in other words, you usually want an impression that goes deep enough as a shallow impression won’t have good retention or acoustics. The larger aspect of the mold that fits the outer portion of the canal, bowl of the ear area really doesn’t do much for seal or retention.
Really is no secret trick though, basically just look at the impression and see if it matches basic anatomy and if it seems too narrow, shallow, has too many voids etc, redo it.
They laser scan the impressions and software 3D maps the usable area inside the mold for the electrical parts and how they can be laid out, and often the software can make a guess if there is a bump in the canal or just a void and fill in the gaps, indentation of hair etc. this often used to be done by hand, just spreading a little impression material on the mold, ala patching a wall or seam, but mostly everyone does it in software to my knowledge these days.
Main things are jaw open and/or closed impression and make sure they have decent depth on the canal. They can always shorten what they don’t need for a good fit but a shallow or otherwise bad impression will simply lead to a loose fitting mold that leaks sound, works out over time etc.
Impression material is pretty cheap so no reason not to just redo another one, spending an extra few minutes and like $1 in material opposed to have a poor fitting in-ear.
They can often average several scans of the impressions too, so sometimes sending mouth closed, mouth open etc can allow the software to get a good understanding of how the canal changes and take all that into account.
Eventually I think everyone will move to just putting a scanning probe into the canal and it will 3D map the canal directly, so no need to squirt the impression stuff etc, but those systems aren’t too common place and expensive
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