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.
Many students find concentration to be the hardest spellcasting component to perfect. Speaking the incantation, conducting the wand movements, focusing upon a target or thought, and having enough willpower are all important for the spells that require them, but managing all of this and then concentrating upon a thought, feeling or desire after the spell has been cast is usually very taxing. Concentration when casting the Dancing Lights Charm, for example, requires concentration on the effects of the spell (the orbs of light that are created). Other spells may require concentration on some symbolic effect of the spell (such as happiness for the Patronus Charm below), and others still maintaining focus on the original target. Concentration is essentially a way to continue feeding magic to a spell, keeping the spell alive and directing it after the initial casting.
The best way to discuss concentration may be to look at the spells that require it. The most well-known charm that has this requirement is the Patronus Charm. The charm is meant to repel dementors, but it requires that the focus upon a very happy, very powerful memory before casting the spell (see Ch. 8 for more information about the "focus" spellcasting component). The memory then shapes the magic to create a non-living embodiment of happiness that a Dementor cannot handle, but the spell will only remain in place as long as concentration on the happy, powerful memory is maintained. Thus, concentration can be summarized as "focus over time” or “continued focus after a spell has been cast”.The Floating and Levitation Charms also require focus to be maintained after the spell is cast (i.e. they require concentration). While the required concentration is low, focus must be maintained on the object after casting these charms. Maintaining more than minimal concentration makes casting another spell simultaneously extraordinarily difficult.
In many cases, a spell's focus and concentration components are not the same, as they were in the Patronus Charm. The Tattoo Charm you will learn about in third year, for example, requires you to focus on the target you wish to tattoo but concentrate on the appearance of the tattoo. You must begin visualizing the tattoo immediately after casting the spell. This concentration must be maintained from the moment you cast the spell until the tattoo is fully formed; losing concentration before then would cut off the magic and leave you with an incomplete tattoo.
Some spells, like the Severing Charm, are "one and done" spells that require no sustained magical energy since their effect is instantaneous. Others, like the Locomotion Charm and Softening Charm, require very little concentration after casting; their continued operation depends primarily on the focus and willpower you charge them with at the start, which also means they cannot be sustained indefinitely with concentration.
As a general rule, concentration is required when ALL three of the following criteria have been met and NOT required if any criterion is not met:
The spell’s magic is sustained beyond the initial casting; AND
The caster or the caster’s wand is NOT touching the target; AND
The spell can be ended at any time without a counter-charm.
As demonstrated, concentration can mean several things. While it usually appears only in more advanced magic, there are some good practice spells the beginner witch or wizard can use to understand the required mindset. These spells, modifications of the Wand-Lighting Charm, can be found in Chapter 17. Understanding how they work will help you significantly when casting such spells in the future.