Stress grants Rikiya the ability to convert stress, anger, and frustration into raw power. The raw power is capable of increasing his strength and size.
The higher the degree of stress, the larger and tougher the user can make parts of their body. The sheer power gained from this ability is represented by black markings that spread to the enhanced areas of the user's body.
Any limb can be enhanced at will, or the user's entire body can be improved all at once. A lone finger was enough to destroy a Double of Twice and a single hulking arm annihilated several League of Villains clones with a single swing.
Rikiya can adjust the percentage of "Liberation" in his Quirk as a measure for his overall enhancements. 80% transforms Re-Destro into a powerful, black, hulking monster that far outclassed Tomura Shigaraki's physicality. 
Because Stress requires the user to experience unhealthy emotions to work properly, more positive feelings, like peace and relief, can end up countering its effects by diminishing Rikiya's own levels of stress.
After being forced to wear prosthetic legs, the artificial limbs do not sufficiently support Rikiya's weight and power, preventing him from using his power to the fullest, as seen when he's forced back by Tsukuyomi's Ragnarök.
Re-Destro has mastered his powerful Meta Ability thanks in part to the burden of the history of Meta Liberation and the responsibility to build a liberated future. This repressed stress has resulted in Rikiya's male pattern baldness and his tendency to be over-emotional.
Stress is generally used to reinforce Re-Destro's body with black markings. This grants him superhuman strength and speed while increasing the overall mass of his body. Re-Destro utilizes his stress according to the strength of his opponent. He only needed to use a single finger to destroy one of Twice's Doubles, but resorted to transforming his entire body to take on Tomura.
Re-Destro uses the strength of his hulking form to overwhelm his opponents with brute force. In this form, he possesses incredible strength that perfectly counters Tomura's Decay Quirk. Re-Destro can pummel the disintegrating ground with a single strike, his hulking frame belays Rikiya's maneuverability. Easily able to catch up to Tomura afterwards and crush the fingers on his right hand with just a pinch of his own giant index finger and thumb.
While using 80% Liberation to increase his enhancements, Re-Destro monstrous form can even attack with the black stress energy. He was able to strike Tomura with a hand full of stress that sent the latter flying across the battlefield.