You'd think it was so simple that it wouldn't even be a thing. Plus I think maybe I've written about it before, particularly about the subtlety of working with the board horse just going in and out of the fields, especially at the gate or at the stall, waiting for them to put their halters on. Or maybe I just thought about it, I don't know.
I do know I wrote about this tho -- how humans want to be effective, and if you start at effective, you never ever ever achieve light (and in fact, you lose effective but let's not go there).
So I've bridled horses all my life and I'm not bad at it: I don't bang teeth or get much resistance. But except for the draft horses who have pretty much already been trained to open their mouths for the bit (and I don't really know how this was/is done), I've pretty much always stuck a finger in the side of their mouth and guided the bit in.
And then from watching or listening to something, I have no idea what actually, I started waiting. Buck has some really good explanations on how to bridle and I basically start like that, but when that bit is held flat in my fingers, I just stop there and don't put my thumb in their mouth. I likely touch their lips with the bit. And lo and behold, much to my great surprise, with no sugar cube and no molasses on the bit, they open their mouths and take the bit in.
And so there that is.