The phrase refers to the process of letting a massive amount of fart gas out in a carefully metered way- usually by farting silently while walking. In this manner, a disasterously huge amount of fart may be discretely dispensed over a larger geographical area. When faced with the prospect of needing to fart in a socially awkward situation, a person often resorts to initially letting a test fart. If the test fart indicates a vile, deadly amount of gas, the person may opt to walk (for example) from the punch bowl across the dance floor, and over to the bar- all the while silently farting the whole way. People at the punch bowl will begin vomiting, the dance floor will clear and the bar area patrons may begin to pass out. Meanwhile, the farter may actually be dozens of feet away, thus escaping blame. In such a way, the farter is said to be "spreading joy."
Damn, Frank is such an idiot. He farted over by the band and kept walking. He's spreading joy all over the wedding reception. How disgusting!