When you hit this button, a DJ pops out of the glove box and a 50 ft disco ball drops from the sky. This is also why the new 4Runner is boxy. Its a throwback of an old 70s speaker system
It says in the manual that the the speakers on the rear doors change over to the two speakers located on the rear lift gate when "partymode button is pressed.
It gets a lot deeper and louder on the bass in my opinion...it's a kickazz stereo and as dsoon as you get in and turn the engine on the bluetooth(I have an iphone) hooks up to ipod feature of the phone and starts playing....really cool