Synthetic Battery Overdischarge

Sapient Synthetics have three main "organs", as it were. The central fusion reactor, the brain-equivalent computer systems, and the batteries. The reactor provides the power, the brain is, well, the brain, and the batteries store power. This is mainly for in case the fusion reactor has no fuel, often due to being in a vaccuum or very thin atmosphere. Their capacity depends on the synthetic themself, but overall they can usually power one for a couple hours, most commonly 5-10, usually enough to get plugged into an electric grid. But while these batteries normally only begin discharging when the reactor falls silent, they can also be forced to do so, essentially supercharging the synthetic doing so.  

Mechanics

The batteries that store several hours of power are rapidly discharged into the synthetic's system, boosting their abilities while damaging them at the same time. This can be done in different levels, the highest of which can turn an ordinary person into a terrifyingly fast and strong fighter while shredding their body.   The main effects of the energy boost are an increase in processing speed, reaction time and general coordination, as the brain-equivalent and signal network are pushed to preform more and faster. In addition, the increase in energy and power pushes the artificial muscles synthetics use to move to function past their safe limits, granting a significant boost in speed and strength.   Unfortunately, this surge of electricity and pushing past safe bounds causes damage to the body, usually proportional to the size of the boost. Artificial muscle fibers tearing while moving, electric signal pathways getting worn, and the body generally overheating. This is why the ability is as rare as it is, and also why it's mostly a desperation manouvre if used.   The ability also often has a side effect on the user's mind. Usually very minor, but it can interfere with some neural pathways and such, slightly affecting the user's personality or otherwise influencing them. Quaken, for example, entirely loses the ability to vocalize speech, and Chaer becomes considerably more sadistic and bloodthirsty, a contrast to his usually stern and calm demeanor.  

Rarity

While technically a voluntary action, most people with the ability to do it have some mental block against it. Most synthetics manufacturers also tend to install physical blocks into their products to prevent them from accessing such a potentially harmful ability. These blocks are usually both hard-coded and mechanical, making them very difficult to remove. It can be done, like in Chaer's case, but it's not common. Some don't even have these blocks, like Quaken.  

Visual Changes

As well as the drastic performance improvements, activation of this ability brings with it some distinct cosmetic differences, which reflect the increased energy coursing through the user's body. There's two main ones. The first is that the eyes of the synthetic, if those are present, light up more, and even more depending on how fast the energy is discharged. The second one is the presence of electric arcs along the synthetic's body. Despite being, well, electric arcs, they are suprisingly harmless, gliding across the plating of the synthetic from more open sections, often joints and the like. At lower rates of discharge, they are generally only present in the legs and near the ground, but as more energy is discharged at a more rapid rate, they start to appear more and over the entire body.