A double shine (or multi shine, triple shine, etc.) is the repeated use of Reflector as an attack by either Fox or Falco in Super Smash Bros. Melee. 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. It is a very difficult and technically demanding technique and is often practiced at slower speeds in training mode, though it is easier to perform with Falco as his jump squat animation is 2 frames longer. Performing more than two shines in a row in this way is known as "multi shining".
The double shine and multishine is known as an effective pressure tool, especially against shielding opponents, however, Fox players rarely use the technique in tournament as it is extremely technically demanding. Multishining is often used in a non-tournament setting as a demonstration of technical skill. In gameplay, Fox players will often use a double shine in order to beat out options such as shield dropping or spot dodging, as the second shine increases the likelihood of hitting the opponent with either shine. Hax is one of the few Fox players known for being able to consistently use multishines in tournament, often as shield pressure.
Because multishining is significantly easier to perform with Falco, Falco players frequently use multishining as a powerful form of shield pressure or as a combo starter. Falco's shine does more damage than Fox's and also doesn't push the shielding opponent as far horizontally, allowing Falco to quickly cut away at an opponent's shield. This can cause opponents to get shield poked or allow Falco easily land a grab. When performed optimally, a multishine can also hit an opponent out of a roll; even if the opponent manages to roll out, it give Falco the opportunity to convert off of a roll read.
Note that frames given assume that inputs are given during the first frame of every range (i.e. all inputs are frame-perfect). It also assumes none of the shines hit (i.e. does not count freeze frames).