A kickback reaction is a collision of two gliders resulting in a single glider travelling in the opposite direction to one of the original gliders. This is important in the proof of the existence of a universal constructor, and in Bill Gosper's total aperiodic, as well as a number of other constructions.
Besides the 90-degree collision shown here, one other two-glider collision with gliders meeting at 180 degrees also produces a clean output glider, and may also be called a "kickback reaction". This reaction is occasionally useful in glider syntheses. However, it is rarely used in signal circuitry or in self-supporting patterns like the Caterpillar or Centipede, because 90-degree collisions are generally much easier to arrange.
