The casting arc could be defined as the angle through which the rod-butt pivots from the beginning of the cast until the initialisation of the stop. Some might say that it is the angle that the rod-butt pivots from the beginning until the end of the cast but this is not specific enough because the fly rod can come to rest at any angle depending upon the size of loop required.
To maintain a straight line path of the rod-tip and hence the fly-line the casting-arc has to match the flexing action of the fly-rod and adjustment can only be achieved by the caster between the start of the cast and the intialisation of the stop, therefore the two are inextricably interlinked for this reason.
A narrowing or widening of the casting arc can cause the rod-tip to deviate from the straight line path and cause a tailing loop (narrow) or an open loop (too wide). The most common cause of a narrow casting arc is creep, where the fly rod is moved forward prematurely before the cast is made.
By narrowing the casting-arc this can cause the caster to accelerate the rod-tip at a much faster rate, to achieve the required rod-tip velocity to carry the fly-line forward and turn it over completely. As a consequence the fly-rod will flex much deeper, which will have the same effect as narrowing the casting-arc, just before the stop is initialised.
A narrow casting-arc will create a concave rod-tip path and as a result a tailing loop will form, whereby the line-tip passes under the belly of the fly-line, resulting in a collision or so-called wind knots.
Using an excessive casting arc will cause the rod-tip to follow a convex path and this will result in an open loop.