This also reflects of what Sullivan has shown me and said. Interesting about carbon...didn't know that.The way I understand the formation of the PO image in pt/pd, cyanotype, and even carbon (though you can't see it for the pigment -- but can be seen if you leave the pigment out...info from Sandy King), is that the printing out image does not form instantly, but takes awhile to form (info from Terry King). Thus taking a longer time to reach a certain level of exposure allows for more of the PO image to form during the exposure. A brief intense exposure, but at the same level of exposure, there will be less PO image formed during the exposure (but I am guessing the same strength of PO image will form, but mostly after the exposure is over...as the PO image formation has a little lag time).
The PO image holds back the shadow exposure a little, and also holds back the mid-tones, but not as much as the shadows -- and the highlights not at all. This can give a smoother gradation between the shadows and the highlights.
To improve the tonality of some small PT/PD prints that only required a 6 minute exposure, I have given the print two to three minutes exposure, removed the print from the light to allow the PO image to form, then returned the print to the light to finish the rest of the exposure. It seemed to help...but this is a subjective test, not quantitative by any means.
I have read that for smoother tonality in salt prints to start the exposure in the sun (perhaps to kick-start the PO image and build up good shadow density), then finish the exposure in open shade.