Argent
We know that you can't Lash people in Shardplate, but can you Lash the person inside the Plate? If they had their helm off, for example. At that point Plate should be just dead weight, right?
Brandon Sanderson
There's a bit of an interference envelope. Wearing plate, the person has this big ball of investiture around them, and so pushing any through it--even by touching a person without a helm--is going to be tough. Easier than with the helm on though, I suppose.
Investiture acts (roughly) like a saturated solution in these cases. Sticking more power into something like a Feruchemical storage or a hyper-invested object like Plate is increasingly hard. The other part is that Investiture tends to interfere with other Investiture, unless there's a familiar resonance. (This is part of what philosophers call Identity.) Slapping your hand through a sand master's stream of sand will cause interference, and make them start to drop. It's not that the sand is supporting them, it's that the investiture holding them up gets scrambled for a moment because of your own investiture.
Investiture pushed toward someone inside a hyper-invested (supersaturated) system like a person in Shardplate is going to get hard push-back.
This is similar to the reason that it's harder to Push on invested coins. Depends on how invested they are, in that case. It's generally not as hard as doing something like Lashing a person in plate. (This is more about the interference than the saturation of investiture.) But the two principles are what I use to guide the physics in these areas.
quietandproud
Can we take that as a hint that the Investiture in the Plates and the Investiture that the Surge of [Adhesion] uses come from different Shards? Or do they interfere because they "belong" to different spren?
Brandon Sanderson
You know, I should have realized this one would bring out the follow up questions. Let's leave it at what I posted for now. This is a deep, deep rabbit hole, and I do need to try to get some more writing done tonight. So...RAFO. (Sorry.)