According to Peng et al (2009), during male ejaculation, the pelvic muscles undergo coordinated intense contraction to expel the semen out of the male genital tract, a process associated with locally increased levels of lactic acid and free radicals as byproducts.

So I think, one of the ingredients of Durex Play O which I've noted on the packaging - lactic acid; may play a role in enhancing the whole stimulation process. Normal lubricant does not contain this.