Most couples will wish to have a photograph of their first dance. The format for this varies somewhat but is typically scheduled as the lights are turned down after the wedding breakfast. The bride and groom are called to the dance floor, perhaps by the leader of the band, and are expected to dance the first number alone on the dance floor. All the assembled guests watch, cry or take photographs.
How is a wedding photographer to capture this iconic couple of minutes? Ambient light may be very low, disco light may be flashing all over the place, and the couple themselves are inevitably moving across the dance floor and possibly rotating as they go. The safest way to be sure of getting a worthwhile image is to fire an on-camera flash either directly at the couple of bounced off the ceiling. This will freeze the movement but remove much of the atmosphere created by low ambient light or coloured disco lights.
Another approach, made more practicable by the arrival of digital cameras with excellent low-light performance, is to select a very high ISO rating and work with available light. Even if the motion of the couple cannot be frozen, the effect may be much more atmospheric than that obtained when working with flash.
A third and very attractive technique, made possible by the support of a second photographer, assistant or lighting stand, is to use an on-camera flash to trigger a second flashgun held behind the couple. The second flashgun fires at the same time as the on-camera unit and highlights the dancing couple from behind. If the second flash unit is triggered by an infrared beam, there must be a line of sight link - which may be difficult to achieve whilst holding the flash unit out of sight behind the couple. A better arrangement is to use a radio-trigger so that line of sight is not necessary.
Given time (typically two minutes!) try all the techniques.