After enduring a humiliating defeat against Androids 17 and 18 in Dragon Ball Z's classic Android Saga, the various Saiyan characters scrambled to find a way to push themselves beyond the normal limits that came with the Super Saiyan combat transformation.
This quest for ascension was intensified in the subsequent Imperfect Cell Saga, as an even more powerful villain was introduced to menace the Z Fighters, with Vegeta and Future Trunks leading the charge. And when Trunks finally confronted Cell after the synthetic enemy reached his dreaded, perfect form, the time-traveling hero unveiled a power that some fans have confused with an early iteration of long-promised Super Saiyan 2.
For much of DBZ's Perfect Cell Saga, Vegeta and Future Trunks trained diligently in the Hyperbolic Time Chamber for a relative year within the span of a single day, given the time-bending nature of the Lookout's pocket dimension. Emerging significantly stronger, Vegeta unveiled a combat transformation that appeared to exceed that of a typical Super Saiyan, appearing visibly more muscular and easily overpowering Semi-Perfect Cell. Yet as Future Trunks watched, horrified, his father allowed Cell to absorb Android 18 and reach his perfect form to provide the Saiyan Prince with a real challenge. Vegeta's hubris led to his defeat at Perfect Cell's hands, with Trunks stepping up to confront the villain next, revealing he had an even more powerful Super Saiyan transformation than his father.
In this new form, Future Trunks was even larger and more muscular than Vegeta's ascended form, with Cell begrudgingly amazed that the younger Saiyan had grown so much stronger in such a short amount of time. Cell admits Trunks exceeded him in terms of raw strength, but the drastic decrease to speed led Cell to emerge victorious. Goku recognized the transformation's drawbacks, including its accelerated consumption of energy while maintaining the form as he achieved it separately while training in the Time Chamber. However, due to Cell's admission that Future Trunks is stronger than him in this form, there has been some confusion if Trunks had quietly breached the powers of Super Saiyan 2.
It is important to note that Trunks' transformation has been dubbed Super Saiyan Third Grade -- compared to Vegeta's Super Saiyan Second Grade -- or even Ultra Super Saiyan among North American fans. However, Goku's observations make it quite clear that this wasn't Super Saiyan 2 at all, but instead channels all the raw energy of the Super Saiyan transformation toward strength and power -- at the expense of its usual boost to speed and dexterity. The full-power Super Saiyan form Goku and Gohan debuted after their own time in the Hyperbolic Time Chamber would push the transformation to utilize its full potential, balancing speed and power in a way that both Saiyans could stand toe-to-toe against Perfect Cell before Gohan unveiled the true Super Saiyan 2 transformation to destroy the villain.
Future Trunks would eventually achieve the Super Saiyan 2 transformation, albeit years later when he resurfaced in Dragon Ball Super. His reunion with Vegeta also saw the Saiyan Prince dismiss the Super Saiyan Third Grade form as primitive. Rather than breaching the barrier between Super Saiyan and Super Saiyan 2, Super Saiyan Second and Third Grade is more a redirect of the Super Saiyan transformation's raw energy toward different competencies, like skill points on an RPG attribute tree. It was an intermediary between Super Saiyan forms. And though Trunks was perhaps stronger than Perfect Cell, this form still paled in comparison to the overwhelming power of the true Super Saiyan 2.