Flicking your Bic
In the early days of Audience Participation at The Rocky Horror Picture Show, theater-goers would hold up and ignite their pocket cigarette lighters during the choruses of "There's A Light." This was referred to a "flicking your Bic", in reference to a long-running (and often suggestive) advertising campaign at the time for Bic brand lighters, "Flick My Bic."
As time went on and Fire-Marshals began to check in on Rocky Horror screenings, theaters banned the dangerous practice and allowed only flashlights. In recent times, it is more common to see people using a smart phone to participate in the "There's A Light" scene.