power actually at work: the active operation of a power beyond the human, producing a real effect rather than sitting in reserve.
Greek had several words for power, and this one names the working end of it. δύναμις is power a person or God has, whether or not it is being used at the moment. ἰσχύς is sheer strength. κράτος is power exercised as mastery or rule. This word is power in operation, which is why Paul can stack all four into one sentence about God raising Christ without repeating himself (Ephesians 1:19).
All eight appearances are in Paul's letters, and in every one the one doing the working is beyond the human. God's working raises Christ from the dead and will remake the bodies of his people. It labors inside a servant so that his exhausting effort actually accomplishes something, and it holds a whole community together as it grows. In Second Thessalonians the same word turns dark: once for the hostile working behind the lawless one, staging signs that are lies, and once for a working of delusion that God himself sends on people who have refused the truth.
The range holds together because the word itself says nothing about who is working or whether the outcome is good. It says only that a power is in operation and that something is really happening. That is what makes Paul's warning in Second Thessalonians 2 sharp: what the lawless one puts on display is not a stage trick but genuine working, from the wrong source. Paul also uses the matching verb ἐνεργέω, and in Colossians 1:29 he sets noun and verb side by side, so that God's working is what is working in him.