You have taken away uke's support and gravity takes over. There is nothing left for uke to do but go down. Moving forward as you go down won't work because uke has time to adjust. The downward side of the angle must be straight down, not angled down. The last topic in our discussion of aikido geometry is circles.
Circles show up a lot in aikido.
Most every movement we make in aikido has some circular component. I want to talk about spinning concentric circles and why we need to understand them. If we look at a small circle inside a larger one, it's pretty obvious the distance around the largest circle is more than the distance around the smaller circle. This means when both circles are spinning at the same rate, a point on the edge of the larger circle must travel a greater distance in the same amount of time. So if nage can become the center of the circle, uke becomes the outer circle.