One of the most memorable moments in all of Dragon Ball Z was the showdown between Goku and Majin Vegeta, with the Saiyan Prince allowing himself to be empowered by Babidi's dark magic to transform and close the growing gap between the two longtime rivals. What resulted was a more evil, uninhibited Vegeta that gave Goku one of the most harrowing fights he had ever endured as the two Super Saiyan 2s' feud reached violent new heights. While Majin Vegeta remains a fan-favorite moment across the entire Dragon Ball franchise, the question remains: what exactly did Babidi's magic do to augment Vegeta's natural potential?
Babidi's magic took advantage of the darkness festering in his intended targets' souls, bringing out their baser, more sinister nature while enhancing their strength and endurance. Identifiable by a prominent "M" emblazoned on his converted acolytes' foreheads, Babidi's followers were largely under his complete control as he plotted to free Majin Buu from his hidden prison on Earth.
The first two acolytes the Z Fighters encountered were Yamu and Spopovich -- two human entrants of the 25th World Martial Arts Tournament. Watching Spopovich in action, including recovering from a mortal injury and demonstrating the ability to fly, Vegeta learned the warrior was far stronger than his prior tournament appearance, fueling his desire to let Babidi transform him.
One particularly noticeable detail about several of Babidi's disciples is that the more humanoid figures seem unnaturally empowered by the magic coursing through them. Yamu, Spopovich and Vegeta all appear paler and with more pronounced veins on their bodies after being converted by Babidi's magic, while Spopovich lost all hair on his body, in contrast to his appearance in the 24th World Martial Arts Tournament.
This implies that Babidi's magic pushes its recipients' bodies far beyond their usual limits, potentially with long-term health consequences, as a sort of magical steroid that exponentially boosts their potential. However, the Majin transformation is more than just an all-around buff to its recipients -- it also visibly and abnormally enhances endurance.
While dueling with Spopovich during the 25th World Martial Arts Tournament, an increasingly desperate Videl accidentally breaks her opponent's neck with a mighty kick. To everyone's shock and horror, Spopovich is able to manually reposition his neck back into place and resume the fight like nothing happened to him at all.
Later, Vegeta endures a particularly harrowing beating from Majin Buu, which he may not have survived had he not been benefiting from Babidi's magic at the time. This indicates that Babidi's magic truly makes its recipients unnatural monsters who are undeterred by most attacks to the body, with all of his acolytes vaporized to eliminate the possibility of them resuming battle after enduring conventional grievous injury.
Even after training in the afterlife for seven years, Goku still found himself fighting for his life against Majin Vegeta and nearly killed on more than one occasion. While Vegeta didn't appear to gain any new abilities from his magical upgrade, it pushed his already impressive Super Saiyan 2 capabilities even further, making him one of the most formidable opponents Goku has ever known.
Augmenting his power and endurance, Vegeta could not only dish out far more punishment, but take significantly more himself. Judging by Babidi's other acolytes, it appears the evil wizard takes a body and twists it into something monstrous, pushing it beyond the bounds of conventional physiology and upgrading it in his fiendish image as he plots to make the universe pay.