Don't know how to use it? Are you kidding me, The old passive was RNG there is no "how to use it", there is only "pray it kicks in".
The problem was, it only worked on AA in the first place, with a 40% rng that seemed to use a completely different algorithm to crit and it only blocked meaningful damage pre lvl 4-5 and non-existent for the rest of the game.
Too many gatekeepings to make it even remotely useful, that's why it sucked. And we now we got another sucky passive for a change, well one could argue that it's now more useful but doesn't change the fact both passive kinda suck.
First of all, you can look through my post history, you can see that for a fact, I am actually a Riot hater. But why am I defending Sion with my life? Because it's actually a very well executed rework, as I said, this rework and Yi's are the ones I hold the most regard of since they are both done exceptionally well, objectively with everything considered.
I did hate when a Sion rework was announced, and I did try him the instant it came out and was completely dissatisfied by the fact playing him as Trynda #2 is no longer viable and dumped him with anger. 2 weeks later I picked him up again and tried playing him like Olaf with slight changes, and I was completely blown away because I could still be a nightmare to the enemy team WITHOUT resorting his original gimmicky and unreliable styles of "OP for 10 min, useless 30 min" or "useless for 30 min and possibly OP for 10 min".
I can now cc better, I can still kill people and I can still wreck havoc in the middle of the teamfight. The only difference is I can no longer crit 1 shot while somebody is stunned (shield burst one shot is still there though, kinda), but to trade only that for a far more satisfying overall strength and way smoother power transitions I say that's a good trade.
If you're really into the old Sion's toxic (AP) or the pointless (AD) styles, then I am sorry you no longer get that. But as far as Sion is concerned, AP (sorta) and AD are both viable, you just need to build HP now as the carry style is removed but I really don't think it's that much of a compromise to get a better Sion.
He sucked, and was way too fkn situational, he needed the change. And he got it, he is now way happier, he got some mad cc and mobility while he lost 3 parts of old himself, which one is major (ulti), one was not intended at all (AP 1 shot) and the last one that barely even worked (AD carry), it really isn't a bad trade.