When over head casting we mostly have a fair amount of fly line (for example the head) behind the rod tip during accelerating the cast. That part of fly line outside the tip offers resistance due to air resistance (line surface) and inertia (line mass). We then mainly rotate our fly rod in order to create the desired line speed. As a consequence our rod will bend. In case of a soft rod and a good amount of line outside the tip the rod will bend significantly. Many fly fishermen prefer a stiffer rod here in order to have little less bend supporting to control bigger amounts of line outside the tip.
Instead when performing a static roll cast we mostly have very little amount of fly line behind the rod tip during acceleration. That results in little rod bend when using a pretty stiff rod. If we use a pretty soft rod instead we get more rod bend and that matches tip path during acceleration for a wider arc much better. Since little line outside the tip means little momentum (mass times velocity) in the direction of the cast we need to increase line speed! For an increase in line speed we want to widen the arc. Now that matches much better with the softer rods. So for roll casting we mostly* get much better results with the softer rods.
*Mostly? Yes, because there are some excellent fly casters who are able to create a fair amount of line speed within a pretty small arc while using a stiff rod. I have seen very few fly casters being able to do this, but for them a stiff rod did a great job on roll casts as well.
As soon as we get into the dynamic roll casts we increase the size of D-loops. That means we get more line (mass and surface) behind the tip during acceleration and thus creating little more rod bend matching wider arcs little better.
The differences in the amount of rod bend (during accelerating the rod) between stiffer and softer fly rods are not as big as one may think (as Lasse Karlsson proved by slomo vids some time ago). But they are there and choosing a soft rod for roll casts on short to medium distances is a smart decision for most of us.
Last week we have been into teaching fly casting and fishing for asp, carp and pike again. Hopefully you could make some time for fishing as well!?
All my best