Blutz Waves (ブルーツ波 Burūtsu-ha, lit. "Bruits Waves") are an energy source emitted by large celestial bodies such as a full-moon or the Earth (in Goku's case during Dragon Ball GT) when it reflects rays of sunlight.
When a Saiyan absorbs at least 17,000,000 zenos (ゼノ Zeno, unit used to measure waves of full celestial light) through the retina, a reaction occurs in the tail that causes the Saiyans to transform into ruthless Great Apes. If a moon is not present, the Saiyan, if he/she knows how to use it, can also create an artificial moon to allow themselves to absorb the necessary amount of Blutz Waves needed for them to go Great Ape to compensate.
Bulma, while under the influence of Baby during the Baby Saga, creates a machine that can perfectly mimic the nature and effect of natural Blutz Waves. This machine is used to help Baby achieve his Golden Great Ape form, and later is pointed at Vegeta, allowing him to transform into a Super Saiyan 4. The machine can force the transformation even without the tail, but according to Old Kai, this would lead the Saiyan to have absolutely no control over themselves as even Vegeta who displayed mastery over the Great Ape form back when he still had his tail, had lost control upon being transformed via the machine. A similar device existed in various Saiyan attack pods, where they generate a holographic moon that is somehow able to generate Blutz Waves, as evidenced by Goku's attack pod and the adverse effects it had on Gohan when he accidentally saw it while training with Piccolo for the fight against the Saiyans.
When a Saiyan receives extra Blutz Waves, it will increase their power beyond its usual level and also heal their wounds as long as they are receiving Blutz Waves. After Super Saiyan 4 Goku knocks Golden Great Ape Baby out, the Tuffle-parasite infected Bulma uses Blutz Waves in order to fully heal Baby, and also increase his power even further.
Video Game Appearances
In Dragon Ball Heroes, it is implied that Black Smoke Shenron is capable of generating blutz waves via his shadows, as he managed to use his shadows to transform Legendary Super Saiyan 3 Broly into a Great Ape and by extension Super Saiyan 4.
In Dragon Ball Fusions, after ending up in the Timespace Rift, Towa demonstrates an unexplained ability to produce Blutz Waves which allows her to turn Broly into Great Ape Broly. It is not shown how she does this though she is apparently capable of doing so without a celestial body, artificial moon (such as a Power Ball), or machinery. It is unclear if her ability to generate them is magical or not as Towa is both a capable magic user and a brilliant scientist.
In Dragon Ball Xenoverse 2, Fu possesses an unknown ability to turn the tailless God-like Saiyans Goku and Vegeta into Super Saiyan 4, allowing them to bypass their Golden Great Ape forms completely and transform directly into Super Saiyan 4. They mistook the form for a new godly Saiyan form as they had been in the process of obtaining Super Saiyan Blue but didn't do to Fu's meddling though fortunately it occurred in a time rift which was later absorbed by Fu, thus it did not effect the official history.