When I think of redirection of a force I assume a counter force is being applied at an angle to the incoming force as in slipping a punch and applying force to the attackers arm at a right angle to the direction of the punch.
So when you redirect the push are you changing the angle at which the force is being applied to your arm? And can you do that while keeping your arm straight? I would do it by slightly bending my wrist and elbow. With the added benefit of coordinating mind and body (intent) I can reduce the amount of force reaching my shoulder to practically zero. I think of it as dissipating the force as opposed to redirecting it but I suppose both are at work to one degree or another.
From what I have garnered reading Aikiweb posts, the "internal" folks claim that a sufficiently trained person is capable of resisting a full on front push to the chest in natural stance without displaying any outward movement whatsoever. I've never seen it done or fail to be done so I can't say yea or nay at this point. Assuming for the moment that the claims are true, how could you use alignment and structure to explain the feat?
I've tried it several ways. Of late I've been working on a way to do it quite nicely with slightly bent arms.
I believe that you are correct about 'internal' people making this claim. I have never seen this either. I have seen displays of alignment as I've shown in my video. I've also seen demonstrations of redirecting/dissipating/deflecting (whatever we want to call it). I've also seen a large number of novel tricks that can make it look like this is possible. But I've never seen video, or in person demonstration of someone standing square, not aligning and taking (relatively) large amounts of force. There's a lot of hearsay about such things.
Until I've seen it, I can't and wouldn't try, to explain it. We've got to look at everything on a case by case basis.