CP2 Wiki

Absorption is one of four perks that essentially provide a free mastery point at the beginning of the game:

Unlike the other perks, Absorption doesn't count as a mastery and still allows players to take Energy Mastery later. However, only a few heroes can ever use this much energy regeneration, such as Nova, Tychus, or Fury. With Absorption, any time you use a basic ability, it will permanently add +3 maximum energy and and +0.0195 energy restoration to your hero. This effect will stack up to 100 times. After absorption has been fully stacked, you will have gained +300 maximum energy and +1.95 energy regeneration.

All heroes that effectively use Absorption require the Compartmentalization Perk or the Lightweight Mastery to be used effectively. Tychus is perhaps a notable exception, as Lightweight does not affect his Escalate ability, which is the only reason why Tychus takes Absorption. Lightweight is still excellent to get on Tychus though, especially after he reaches his score unlock.

Absorption has also been covered in the 1st part of the official Perk Detail Overview on BlacKcuD's blog. Check it out for more details on how it works, known exceptions and use cases.

Note: The definition of a basic ability is an ability that does not require a Mastery to use. For example, Predator's blink ability is NOT a basic ability because it is only available if we pick the Blink Mastery.