There's a move in Chess, called "en passant" (French for "as you pass" or "in passing") in which a pawn can capture an opposing pawn who moves two spaces from start, by diagonally entering the space that would have been occupied if the opposing pawn had only moved one space from start. (That's complicated sounding, I know - go look it up if you're not familiar with Chess.) The idea is that the capturing pawn reaches out with a dagger or stiletto and nabs the opposing pawn in the ribs while they pass each other.
What's that got to do with synodality?
The devil would like nothing more than to destroy the Church. He can't, but if he can destroy it in the minds of the faithful, then he will have accomplished a lot. People aren't saved by not believing in the Church, so destroying the Church in people's minds leads to the loss of souls.
One of the things that the devil knows would destroy the Church in many people's minds is if the Church were to reverse a significant teaching. For many modern faithful Catholics, that the Church always teaches the truth is not only one of the Charisms of the Church (infallibility), it's the demonstrative Charism proving the reality of the Church. If the Church, for twenty centuries, has never "reversed" a teaching, then it demonstrates that the Church is imbued with some special grace of God that prevents it from proclaiming error. Otherwise, it would have proclaimed some error that it would have had to reverse.
Many priests, and even bishops (including cardinals), are bent on getting the Church to reverse its teaching on one matter or another. For most, it's a matter pertaining to sins of the flesh, particularly contraception and homosexuality. But for many, it's the ordination of women, or abortion, or some other matter on which the Church has stood firm for two thousand years. It's likely that the devil thought he had it "in the bag," so to speak while Pope Paul VI was writing the encyclical Humanae Vitae. Well, what a surprise! By this point, the devil probably realizes that he cannot successfully attack the Charism of infallibility, as enjoyed by the magisterial hierarchy itself - at least not directly.
So, the devil has a new plan, and it's called "synodality." The idea is to get those "in charge" in the Church to alter its structures so that "teachings" are no longer coming strictly from the magisterial hierarchy of the Church itself. Instead, lay people would now be included in the process of developing, voting on, and promulgating "teachings" of the Church.
This - lay participation - is the two-block move from the starting line of the devil's pawn.
But it's also exactly where the Holy Spirit will counter by killing the Church' seeds of destruction before they get started. Here's how: If the "synodal Church" comes out with a teaching that (say) contraception is no longer forbidden in marriage, then the resolution of that fact to the long-standing belief in the Church' infallibilty is that this teaching isn't actually coming from the Church. Instead, it's coming from some synodal process, in which hierarchical members of the Church are participating, but is not itself magisterial.
To be sure, the "synodal Church" will be used by the devil to cause the loss of many souls. But we need not fear that what comes out of it disproves the Church.