Quintessential Magic: An Introduction To Charms (2nd Ed.)
A useful text for First Year Charms students, Quintessential Magic delves into the basic methods of casting a spell. From incantation to willpower, Wand-Lighting Charm to Severing Charm, this text covers all the basics.
The Severing Charm & The Softening Charm
The Severing Charm
Incantation: Diffindo (‘deef-IN-doe’)
Wand Movement: “V-Shaped down and up, ending with a swish in the direction to cut.”
Focus: The place to cut and the depth of the slice.
The Severing Charm, invented at some point in the 1400s by a seamstress named Delfina Crimp, allows the caster to precisely cut the target they are focused upon. The caster should finish saying the spell while performing a swish in the direction of the cut. At this point the spell is cast, and the target is cut along the desired path.
Of course there are many possible mishaps that can occur. The simplest is, of course, that nothing happens. The thing you are attempting to cut may also be only partially severed. In this case you may have to cast the spell again to finish the job. Your cut could also be a bit ragged as the magic tears through the material rather than severing it.
In other cases, you may accidentally cut something you had not intended. If your goal was to precisely cut a single flower stem, you might find that you have instead sliced all of its neighbors if you aren't careful. If you are attempting to cut a piece of cloth, you might find that you've also cut into the material behind or underneath it. For these reasons it's best to practice this spell in an open area where a mistake means that you've cut neighboring blades of grass and not chopped up your clothing or walls.
With practice it gets easier to determine precisely where to cut, how deep, and to prevent unintended items from being cut. It's worth noting that this spell isn't nearly as effective against animals and other such complex life forms. Despite appearances, living things are made of layers and layers of very small moving things called cells, and the magic of this Charm does not interact well with them.
You can chop down trees or cut plants because their cells are very slow-moving and relatively simple. You may be able to give someone a scratch or papercut, or perhaps even something a little deeper, but doing serious harm with this spell requires years of practice and by this time it is easily overcome with a Shield Charm.
The Softening Charm
Incantation: Spongify (‘SPUHN-jee-fye’)
Wand Movement: “S-Shaped with a small swish half-way through”
Focus: The target object
This spell is much less devastating than the Severing Charm can be if improperly cast. An improper Softening Charm will usually (instead of only occasionally) result in nothing happening. If you don't give the spell enough charge when casting, you may find that the charm wears off almost instantly or that the item you are attempting to soften is only very slightly softened and doesn't bounce when dropped; it will also lose its softness far more quickly.
The charm is also useless against living matter. Attempting to cast it upon a living thing causes nothing to happen to their living cells, though hair, nails and fur might be a little softer for a short time. Other materials, depending upon their hardness and density, may remain unaffected as well.
Metal and stone typically don't "absorb" the magic well enough for a noticeable effect, though an accomplished witch or wizard may be able to temporarily soften these materials if strong enough (depending upon the material, some cannot be softened at all by this charm). Such things are beyond most students of any grade at Hogwarts, however.