I'm familiar with a few forms of ukemi from shihonage. I hope I'm understanding the topic properly.
1) Pin form: Nage goes straight down to kneeling, hand beside foot. Uke bends back to follow with shoulders, not dropping hips. Important: Big slap from high up to take up impact. Should be simultaneous to avoid shoulder injury! I believe the purpose of this 'stylized' fall is to represent a very fast takedown where uke has no time to do anything but keep the chin tucked to the chest (general back breakfall rule) and slap to absorb force.
2) Throw form: Nage brings uke off-balance as usual for pre-takedown form. However, instead of going down, nage simply steps forward, extending through the arm. This brings uke off balance and sends them into a backward roll. When I say backward roll, I refer to a Yoshinkai/Yoshokai-style roll; one arm off to the side, the other hand protecting the head which is turned away from the extended arm, kick backwards as turn over.
Consult better sources than me before attempting either of these; I explain them for academic purposes, not as instruction.