Most of the time in sinlge player mode you can't do the swingset glitch because you fly out of the windshield.
In multiplayer it doesnt happen you can do it anytime.
Most people i've seen or heard say you cant do it in single player.
Well i found that you can.
Call up one of your friends, brucie or roman or who ever. You can do the glitch with no problem if you have a friend in the car. You wont fly out of the windshield.
Hope you have fun with this.
I wanted to post this for people that dont have broadband internet and live in bumfuck no where like i do and cant play multiplayer. Have fun!