Cell powers up a yellow aura that shows the souls of the humans he absorbed throughout the Imperfect Cell Saga. Because of this source of energy, which Piccolo estimates to be made up of thousands of absorbed people, Cell's power increases exponentially.
Cell uses this Power Up when he interrupts the fight between Piccolo and Android 17 on the Tropical Islands. After powering up, Piccolo is speechless at how strong Cell has gotten in a short amount of time, although he eventually protests that it is not actually Cell's power, but the stolen power of the innocents that Cell killed. Cell then proceeds to defeat the Super Namek and easily dominate Android 17, even taking Piccolo's strongest attack head-on without a scratch.