This is when you go to a small shop with a large bunch of friends with whom you had a curry with the night before. And you all simultaneously fart like hell before exiting without making a purchase. This can be done in stealth by silent ones, or for added effect rip them out as loud as you can, watch the invasion unfold as the shop assistants dive for cover.
If you only have a small group, it can be done in a taxi, but has to be just as you're paying your fare.......Then upon leaving the taxi the doors are closed really quickly. The driver then suffers for over-charging.
'lets go to that crap clothes shop on the and release phase 1 of a fart invasion campaign, we'll get the fuckers next week too'
'This taxi drivers sure to rip us off, I feel the need for a fart invasion'