The second branch of the Ascended Super Saiyan form.
In this stage, Saiyans are similar in appearance to the Legendary Super Saiyan form. The skin of the Saiyan may become slightly redder due to increased blood flow, and muscle size is increased to the point of practical ridiculousness. The aura no longer flows upward, but flails outward, with surges of bio-electricity becoming more frequent and prominent. In the series, the Ultra Super Saiyan form was discovered as an advanced stage of Ascended Super Saiyan (itself an advanced stage of the Super Saiyan transformation) that further increased the physical strength of the Super Saiyan form. However, due to this form being so muscular, mass is increased to the point where the user's speed is reduced to debilitating levels. In addition, the Saiyan's energy rapidly depletes due to the effort needed simply to maintain the transformation. Altogether, the negatives of this stage outweigh the positives.
Believing that he surpassed his father Vegeta in strength, Future Trunks ascends to this form to combat Perfect Cell at the behest of Vegeta's defeat. Although he manages to hold his own for some time, he too is unable to defeat Cell because of his inability to maintain equality in speed. Because of his inexperience with these transformations, Future Trunks does not realize that his bulky form would be an inhibitor of his ability to battle, a characteristic that is later noted by both Goku and Cell. Future Trunks assumes that Vegeta is aware of this as well, believing this to be the reason Vegeta doesn't use the form.
Cell displays a similar ability to bulk up his muscles when he mocked Future Trunks for using it. When Cell fights Super Saiyan 2Gohan, he ends up losing his temper and resorts to using his slow, Power Weighted form, which fails to help him win the battle.