As we all know, spells are a combination of words, gestures, and ingredients. One way to invent spells is to decide that each of these serves a different role in each magic system. If we can’t make up our mind, we can have one type of magic use gestures, for example, in one way, and another type use them for another. It’s the divide and conquer approach to not only inventing spells but magic systems. One choice to make is whether gestures, words, or ingredients are the difference between two spells; we might have a single spell to make a pie, with ingredients determining whether it’s apple or blueberry. Or maybe ingredients don’t include the fruit at all and there are indeed two spells, one for each type of pie.
While gestures could mean anything, it seems sensible that they’re used for assisting with the width of spell for the area affected and the distance from the caster. If I only want people before me to fall asleep via my spell, I would control my gesture to indicate them but omit people to their left and right. If I want to affect people who stand within 20 feet of me, perhaps my arm is a given distance from my body but not too far, so that those more than 20 feet away are unaffected. If I want them to fall asleep instantly, perhaps my gesture is quick, whereas a slower motion might cause a less sudden effect and a gentler fall into sleep (and to the floor).
A spell that targets individuals more precisely can be imagined in a similar way. If three magic missiles will depart my fingers, perhaps I point them at the chosen targets. This could be done in succession with three pointing motions or all at once with one or two hands; we can imagine some inaccuracy with this, plus people being tipped off as to what’s coming. Some variants on spells might require contact with the target, such as wanting to make a single person be the victim. Generally, the more people or wider impact area, the more difficult and tasking we can imagine the spell to be for the caster.
Gestures might also indicate the power level due to force with which they’re made. Imagine that we’re casting a wind spell and merely flick our finger, versus a strong sweeping motion with one or both arms. The first might blow out a candle’s flame leaving the candlestick standing while the other might knock over everything in its path. This is not a rule, but it’s another way to decide what is required of the wizard. Viewed this way, a gesture might indicate passion and determination, which might please the source of that power if a god is the origin and they approve of the result; this could, in turn, convince that source to grant greater power to the wizard.
We can also decide that the wizard has harnessed energy and that, to disperse it away from themselves, these gestures are necessary, or the spell goes off in their face. With that in mind, if others know this, they might try to inhibit such a gesture, so the wizard hurts themselves. A bound wizard might be unable or unwilling to cast various spells. Imposing this limit lets us understand when they can and can’t do something.