system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] How does one create a new spell?

We often read about how famous witches in history created spells, which everybody now uses, and we also know that Snape created two spells himself. Now, I don't understand how this is supposed to work: How exactly can you make someone not say a word while twiddling their wooden stick but do actual magic with this?
gpt
First, lets look into potion creation.  What various parts of the potion tend to do is known.  What they will do when mixed (note that how you mix also plays a part) can be derived with some degree of accuracy, but may take multiple recipes to get the exact potion you want.  
  
Spells work similarly, except the "ingredients" are sounds and wand movements.  One can derive sectumsempre from "sectum" which is an ingredient that involves cutting and "sempre" which makes it long lasting.  These are not exact, so much like how you would vary the amount of eye of newt in a potion, you would vary the exact sound you are making until you get the exact right effect.  Variations of the sound create variations of the effect.