Translated from the Latin, Animam Evocare means "to draw out the soul" or "to call forth the soul". This spell, associated with Ralph Stokesey, can call forth the soul of humans and animals in the form of "a bead of pearly blue light" which will rest in the magician's hand; if this bead of light is destroyed, the being from whom it came dies. Jonathan Strange faces a moral quandary he fails to solve when - having once firmly declared his opinion that a magician ought not to kill by magic - he is faced during battle with a situation in which he must do so to save himself .
The cuirassier raised his sabre. Without thinking, Strange muttered Stokesey's Animam Evocare. Something like a bee flew out of the breast of the cuirassier and settled in the palm of Strange's hand. But it was not a bee; it was a bead of pearly blue light. A second light flew out of the cuirassier's horse. The horse screamed and reared up. The cuirassier stared, puzzled. Strange raised his other hand to smash the horse and horseman out of existence. Then he froze. 
We cannot be sure what Strange would have done next, as at that very moment a British cavalryman rides up and impetuously despatches the immobilized cuirassier.
It is very possibly a spell of this sort which John Uskglass uses to thwart the gentleman with the thistle-down hair's intended plan of destroying Vinculus. It seems that at the dreadful moment when the Gentleman hangs Vinculus, Uskglass is somehow able to take the departing spirit into his own keeping. Then later - and this is the masterstroke, the mark of his extraordinary magicianship - he replaces it. John Childermass witnesses the moment. Uskglass plucks "a tiny pearl of light" out of his own mouth, puts it into that of Vinculus - and the result is that the hanged man awakes, gets to his feet, screams a little and then (after drinking some wine) is pretty much restored to his usual self.