The roll cast is a form of spey cast in which the cast uses the water to "anchor" the line and leader to provide the resistance to bend the rod for the cast. The more effective the anchor, the easier it is to cast. I think Jackster was asking how you anchor the cast to make an effective roll cast.
In a true roll cast, you raise the rod and the line must come to a complete stop
"we wait until the tip of the flyline is stationary on the water
in front of us (with) the line is lying to the side of the body"
If the true roll cast is done on grass, it is difficult for the line to anchor the cast. One can make a cast by not
stopping the rod and moving the line faster
than one would if the line were on the water. If the line moves fast while fishing, it could spook the fish. If one continues to move the line through when performing the cast on grass, it is not
a roll cast at all.