The music used in the transformation sequences.
In Ben 10: Evolution, the transformation sequences were influenced by the transformation sequences from the original Ben 10 series, where they show Nick Kane, the current user of the Omnitrix, morphing in great detail.
While these show the Omnitrix becoming a green ball of energy that flies across the screen and slams onto the ground, the background changing, and Nick merging with the DNA of an alien most of the time and then becoming it, sometimes, it shows Nick in the Codon Stream, where he would run to a giant Omnitrix symbol, and when he presses it, green energy would surround him at where he is at, he would start floating, shrink or grow to the appropriate size of the alien, would gain the color scheme, weight (depending on the form), and features of it, and the alien would take Nick's place.
The show features a unique transformation sequence for Alien X, where Nick would be put in space, and be pulled closer to the same giant Omnitrix symbol. Once pressed, he would arrive inside with Bellicus and Serena, and the camera would zoom out of Alien X, in whatever scene, and a green light would fill the screen and then disappear, finishing the sequence.
In a few transformation sequences, though, the first few seconds is same as the Codon Stream sequence, however, Nick would come closer to the Omnitrix symbol, now with a DNA sample of whatever alien Nick's transforming into sitting in the center of the hourglass symbol, and when Nick touches the symbol, a unique scene would appear for the transformation (e.g.: Heatblast blasting fire at Nick, sucking him in behind him, and returning to the scene in question; Thriller Whale spitting water at Nick that sucks Nick behind him, returning to the scene as Thriller Whale; using a motion that fades the screen back into the scene with Nick as the alien that he has selected).
In some occurrences, the series uses a quick and simple transformation similar to that of the ones used in the other incarnations of the franchise, where a green outline glows on Nick (sometimes on a green backdrop), Nick grows or shrinks to the appropriate size of his alien, and morphs into the alien in 5-6 seconds, or the aforementioned green light irradiates from the Omnitrix, and when it fades out after growing too large, whatever alien that Nick has selected comes into play.
An example of a transformation sequence, which comes from an episode of the series:
(TRANSFORMATION: Nick slams down the Omnitrix dial, and the watch turns into a green ball of energy that flies across the screen and slams onto the ground. The background then becomes engulfed with dark green energy and light green orbs. Nick opens his mouth, which develops several sharp teeth, and his arms, torso, and legs then begin to expand significantly. His ring and pinky fingers merge together, and his fingers altogether grow longer. He is then covered from arm to toe with black and white fur, and the camera zooms into Nick's head. Nick's head then fuses into his torso, and the camera spins around Nick, as chunks of gold form armor plating on his upper torso, arms, legs, and back. A pair of blue pants then appear on Nick, as Nick's ears evaporate, and two panda ears appear on the top of Nick's body, with a stripe forming on his upper torso. His eyes then enlarge and become fully green, and his nose morphs into a panda's, and his fingers and toes then become claws. A belt with the Omnitrix then comes out of the pants and sticks on to the pair. Cannonbolt then does a quick roll, and then uncurls himself and poses.)