The Spiderweb is achieved through male masturbation in the shower, typically done with the aid of a soapy substance. After ejaculation, as the male lifts his hand up and spreads his fingers apart, it looks like a spiderweb. May also be flung at or on partner after said ejaculation in conjunction with the male shouting "Spiderweb!" and mimicking the motion of Spiderman shooting his spiderweb in the popular comic books and movies.
I'm sorry I just Spiderwebbed you in the face, neck, shoulders, chest, and stomach.