He had -15 on the board. Interesting psychological game

The truth is the complete opposite of this. The engine shows you all the possible moves in all the positions, with evaluation. You can look at good moves and/or bad moves, you can try any move you like, and see what happens, and you can go back and forth as you wish.

Uhhh so I'm not sure I'm explaining the root of my confusion; is "advantage" a metric determined based on fractional possibilities such as x/# different possible lines that lead to mate after that move is made in that CURRENT board position?

i.e. 'this move leads to 18 others that leads to 8 others that leads to THIS CHECKMATE--- ***IF you play it perfectly against a perfect opponent?***
i.e. many lower skilled players can often 'land themselves' in such board positions, without any awareness, and then make moves executing on their own ideas, that then "blunder" their *potential* advantage, that they never knew or cared that they "had"?

.....I'm not even saying the computer was "wrong"; I just want to know how exactly it determines what's "right"(in terms of move by move -+ evals as well as accuracy). I'm just speaking for myself, and of course it's my fault for not being a perfect accuracy player, but that ridiculous += eval bar does this kinda frequently, especially if I'm playing at my peak 4 am hours, and against relatively skilled opponents. This doesn't weird other people out at first too? lol 
***I'VE DEFIED THE GODS AND DONE THE IMPOSSIBLE***
......(stockfish), no you've just defied the God's... and you will be punished for your mistake#~~~
*quits*

Absolutely not. Nobody knows that, today's computers are way too weak to calculate an average position to mate. Even in the future, when computers are a quadrillion times stronger, it will still be completely impossible.

And no, weak players can't hack chess by playing too bad moves. What you experience is that you make an obvious blunder and then your opponent replies with another blunder.

The way to get better would be to identify and understand these blunders, and try to not make them again. Instead, you try to explain to yourself that you played just great and the computer can't understand your brilliancy.