Pushing Rope is when you are banging a girl all night, either repeatedly or multiple times, and finally your rod craps out on you. Instead of thrusting with your best foot (member) forward, you are instead pushing a limp noodle, hence the term, you are pushing rope.
This can also be referred to the term "whiskey dick", full sail, no wind (as you have full intention, but no control of whats going on)
Suzy and I were so into it last night, we were banging till I was pushing rope!
Buddy, I was so wasted last night, I was pushing rope.