How does ddavp (desmopressin) work?

Reduce urine amount: It is a synthetic anti-diuretic hormone. Acts on the kidney to reabsorb more filtered water so that less urine is produced. Idea being that the bladder will be able to accomodate a smaller urine volume produced and not reach a threshold where nervous system feels it has to expell urine, whilst subject is sleeping, and result in control of bedwetting.