Deploying a backup or reserve light is done on sound principles.  The first principle is that if you have lost your light you need to get another on quickly.   Thus pop the head out of the retaining bungy and turn on the light so that you can signal - do not unclip it at this stage!

Once our team have seen that we have a problem, we can stow our primary light by the top clip.  The second principle of the drill is that we must ensure that our light cord is properly tucked away so it won't snag as we swim back but above all we must ensure that it is tucked under the long hose so that it doesn't foul donation of gas.

The final principle is that we only remove our reserve light from our harness when it is on.  If you drop a light which is switched on you have some chance of finding it (if there is a bottom to catch it).  Unclip and signal that all is well to the team and that we are ready to leave.

To end the drill remember that the same principle applies.  Clip off to your harness and then switch off - with a pressure check to ensure that the light really is off.  restow and deploy your primary once again.