As the name suggested, the spell charged a gem with a portion of a soul stolen from a subject for an entire day, the subject could die from it. Due to the long preparation time, the spell was almost useless for use in the midst of combat.
The gem or the energy in it could then be used to substitute for the charge of a magic items, but it had some limitations. Apart from using a gem high value, one gem could only store enough for one charge, though nothing prevented the caster to hold multiple gems with each storing a charge. And even a charged gem could only substitute a charge of a spell with a limited power limit and only one that the caster could at least potentially cast without the help of an item.
Soul charge used verbal and somatic components but also required a focus, the gem to be charged.