Tell me if I'm wrong, your initial values are 25 Opp and 10 You, after all it's ended things are: 18 Opp, 25 You. Your 7 comes out from the 25-18=7 right? (that's what I understand for "net gain"). (if this is right then keep reading, if it's wrong, skip the rest of the post and explain better where your 7 comes from)
But if you change the initial values, like Opp 60, You 30 --> Succubus steal 15 strength unit --> Opp 53, You 45, and you apply the same math: 45 - 53 is -8 which doesn't make much sense to me (referring to -8 as ''net gain'').
With how i mean ''power swing'' initial and final values are meaningless, because the power swing just refers on how strength shifted overall on the board from your perspective.
I'll try to explain again: let's take initial values of 25 Opp, 10 You, let's imagine that the succubus effect is immediate, so no other cards can be played. I play succubus on opponent's side: 33 Opp, 10 You, --> succubus steal 15 strength --> 18 Opp, 25 You. (so the power swing is 22 because the opponent lost 15 (stolen unit), I gained 15 (stolen unit) and opponend gained 8 (succubus) using the simple equation I written above, 15+15-8=22 )
(related to the initial values of 25 and 10 mentioned above) Initial difference of strength is -15 (you have less strength than the opponent), final difference of strength is +7 (you have more strength than the opponent) now if you subtract from -15 the final difference (+7) and you take the absolute value you get: | -15 - (+7) | = | -22 | = 22 which is the power swing I already mentioned multiple times.
This works in the same way with any initial value: 60 Opp, 30 You --> You play Succubus --> 68 Opp, 30 You --> Succubus steals 15 strength unit --> 53 Opp, 45 You.
The initial difference is -30 (the minus it's there because from my perspective I have less strength than the opponent). The final difference is -8 (the minus it's still there because I still have less strength than then opponent). As above now I will subtract from the initial difference (-30) the final difference (-8) and I'll take the absolute value: | -30 - (-8) | = | -22 | = 22 which is again the same power swing I mentioned before.
To summarize, if the card Succubus, with a base strength of 8 at this point in time, steals a 15 strength unit from the opponent's side, the "power swing" made by this card is 22 regardless of any initial and/or final values of ''side strength'' (the total strength of the individual sides of the board), in other words, 22 represents the absolute variation of strength in relation to the difference of your side total strength and your opponent's one.
If I sound too complex it's probably because I'm studying engineering, the actual calculation is fast and simple, however I do not know how to explain my definition of ''power swing'' (which I believe is the same of Tamacountry13) any better.