Natural corks take longer to decompress and return close to their
original diameter and the pressure created will leak out over time if
you leave the bottles upright./ That is why the advice is to store
them upright for a day after corking. Synthetics bounce back
immediately and seal the bottle as they are inserted making them the
piston and the bottle the cylinder. They will never leak any of the
bottling pressure out and I have always wondered if that is the reason
sulfite levels measured in a synthetic corked bottle are often lower,
they have more oxygen to scavenge. Sumpremecorq recommended either
vacuum bottling or the fishing line trick for homewinemakers. If you
use a decent test line and pull it slow it could work. String
trimmer line might work better. I use Nomacorc and don't bother, I
have wines under them for 3 to 4 years and have no complaints.

As an aside, anyone with a brass jaw corker needs to be careful with
synthetics, you need to examine the seal area closely to make sure
your corker is not scoring the corc and creating an imperfect seal.
Natural corks are much more tolerant. If you are getting scoring the
jaws can be adjusted. Iit's a pain to do and the jaws are under a lot
of pressure, if you are not mechanically inclined you shouldn't take
one apart.

It's good practice to leave 1/2 to 3/4" headspace on wines corked with
anything to allow temperature changes to be absorbed within that
headspace. A corked wine bottle is a big thermometer....

Joe