A double shine (or multi shine, triple shine, etc.) is the repeated use of Reflector as an attack by either Fox or Falco. It is performed by jump-cancelling a Reflector and using a second Reflector just after leaving the ground, timed so that Fox or Falco will quickly land and allow the pattern to be repeated.
(Down + B) , (Jump) , (Down + B)
A series of frame-perfect multi shines at 1/4 speed.