Ukemi (受身) is a side roll when hit onto the ground. It can be performed by pressing LP or Down+LP. Ukemi is the waking-up equivalent of sidestepping. It can be used to avoid an okizeme as back rolling can leave the player particularly vulnerable sometimes. During the animation of rolling to the side, the character is actually invulnerable until they are back on their feet.
In its debut, the opponent cannot actually tech-roll if they were being hit from the side or if they are facing downwards during a combo. In Tekken Tag Tournament, while it was possible to tech roll when facing downwards, it would still be impossible if hit at a low angle while facing downwards. By Tekken 4, tech-rolling's speed and safety was improved and could now be performed after being hit by almost any attack except those that spike an opponent downwards, like the Tsunami Kick.
Several throws like Nina and Anna's Jumping Flip or King II's Giant Swing can be tech-rolled as the player is landing on the ground for a quicker recovery, though the Giant Swing, and Armor King II's Double Arm Swing are the only throws where damage can be reduced by tech-rolling.