"stretch the limits" - to go further (in any field) than anyone or any group has done before. perhaps doing something that wasn't previously thought possible.
dude, that quadruple-backflip double-twist one-handed going backwards while blindfolded and sucking a lolly (deep breath) surf manoeuvre was really pushing the envelope.
A sex act in which one person is tied to a bed and a sheet with strategically-cut holes placed over them, through which a second person has sex with them.
"Looks like it's time to buy new sheets. The wife and I were really pushing the envelope last night."
