The Grandstand can be described as the action of having sex with a MILF while their kid(s) are sitting on the headboard, dresser, or bed table watching you two go at it.
This is quite a feat to accomplish and if you can accomplish it, you are considered to be immortal.
Guy 1: Yo, what were you up to last night?
Guy 2: Oh man, I was doing The Grandstand with that MILF from down the hall!
Guy 1: Damn dude, your the fuckin' man!