The Power Ball is a technique used by Saiyans to mimic the presence of a full moon. By combining ki with the planet's atmosphere, a ball of condensed Blutz Waves is created. Using this technique, a Saiyan can achieve the Great Ape transformation even when a full moon (or similar planetary body) is not present. In the first battle between Goku and Vegeta in an earlier dub of Dragon Ball Z, Vegeta revealed that Bardock, as a scientist, invented the Power Ball technique.
It should be noted that some characters (Saiyans with tails) in the Budokai Tenkaichi series of games can use this technique to transform into a Great Ape at any time (Vegeta, Turles, King Vegeta) while others must be on a stage with a full moon in order to transform (Nappa, Kid Goku, Raditz, etc.)