In the first video on this topic we saw a good time to use chess engines is to check a game you played or witnessed where an opening you "know" (or remember, or have a hunch) is bad/refuted paid off for the underdog (the person trying it) and you don't think the result was just/logical.
We looked at:
1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Qb6? (bad/refuted)
6. e5! Bc5 (what else?) and now the key move is 7. Ndb5! (seen in a recent game Hess-Lapshun).
As we see in the video, 7...Bxf2+? loses horribly, leaving black with 7. Ndb5! Nd5 8. Nxd5 exd5 9. Qf3! (the key computer-recommended resource) and white is "up a pawn" in computer terms. Or, 7. Ndb5! a6 8. exf6 axb5 9. Qf3! and again white is way on top after 9...gxf6 10. Ne4! (10....f5?? 11. Qc3! winning).
In the other line we looked at,
1. e4 d5 2. exd5 Nf6 3. d4 Bg4? is a bad gambit. We checked it with a computer to VERIFY that 4. f3! Bf5 5. c4! is insufficient compensation for black.
Try out these lines yourself - have an opponent, preferably higher rated, try it after you've prepared the main lines with a computer, and see if you can bring home the point!