My understanding is that they just installed a lighter trigger (that is, the trigger itself weighs less so that it generates less momentum on drop -- spring vibrations on drop reduce spring tension/weaken sprigs, hence allowing firing). None of the other changes made during the recall appear to directly affect the drop safe issue -- there is no mechanical "drop safe" block, though they did add an out of battery discharge mechanism (but that appears to be since part of the military submission, not b/c of drop safe issue w/ pre-recall p320s).
Sig has a very good video on their website explaining in detail what is done in the upgrade and all the changes that have been made to the new production models. Not that any of this means anything to me since I will not own a striker pistol but the drop safe issue has been addressed and resolved.
Sig's PR:
https://www.sigsauer.com/support/p320-voluntary-upgrade/The reality -- problem may not occur in most testing, but a lighter weight (overall weight, not pull weight) trigger is the only change that directly affects the drop safe issue -- I *think* the military manual safety likely provides addition protection to troops, but I don't believe that Sig's jerry-rigged "fix" is anything but a CYA modification (truly remedying the issue would require additional modification to actively prevent rather than mitigate occurences):
Effectively, my understanding is that the P320 originate w/ LE procurement specs specifying striker fired pistols. Sig had none, so modified the P250 (a DAO hammer mechanism) to a P320 w/ minimal changes...
The latest reports I'm hearing on striker-fired guns failing are the FNSes -- this after many safety and reliability issues w/ the S&W M&P line as well as Glocks... Taurus earlier had issues, but latest guns appear to have much improved fcg safety mechanisms. Glocks, without all the modifications (and uncertain about Gen4 & Gen5s internal mechanisms) are generally safe ignoring takedown trigger pull.
Also for really light trigger pulls -- and not an exact comparison -- but DA/SA are somewhat similar to single vs 2 stage rifle triggers... There's only so far in one can reduce trigger pull weight before it starts to jeoporadize safety under drops or similar weapon abuse.