This article is about tennis, but chess is briefly mentioned and the links to the comments at Marginal Revolution goes a little further in mentioning our game.
The general idea is that competitors want to maximize Expected Value (EV), which can be understood in a general way by thinking through an example. If you bet a dollar that a fair six-sided die will come up 1, and you’re paid $12 if that happens, then you’ve got a one in six chance of getting the $12. That means that on average, you’ll get $2 per roll. However, you’re spending $1 each time, so your expected value is $1. If the prize is $6, then your EV = 0; in other words, you’ll break even. And if the prize is $3, then your EV is -$.50.
Applied to chess, the thought is that using opening lines that aren’t the engine’s top choice is likely to increase EV. Why? Well, chess is pretty drawish anyway, so even if you play the engine’s top lines a draw is a fairly likely result in any case. Moreover, your opponents are likeliest to have studied those variations, which means (assuming they’re reasonably diligent) that they’ll be aware of both what to do and will have some grasp of what they’re supposed to be doing once theory (or at least their prep) comes to an end. On the other hand, we humans are not very good at chess. (If you don’t believe me, have a session against Stockfish in which you have no computer assistance whatsoever. [*Cough* Kirill Shevchenko *cough*, allegedly.] I’ll wait, but it won’t take very long.) So if we play “off-brand” variations, say, with White that allow our opponents to equalize, it doesn’t mean that they’ll know how to maintain equality or how to play the resulting positions. If they happen to be prepared even for our idea or just have a great day, maybe they’ll achieve the draw more easily than they would have in the main line. (Perhaps the main line ends with a position that’s manageable for Black, but still requires a bit of suffering before the job is done. That’s true in a lot of opening lines.) But more often they’ll be thrown to their own resources far sooner than they normally would, and may lack some of the signposts they’re counting on in main line theory.
But what about the risks of the off-brand approach? It depends on the line and the amount of prep. There are some crazy off-brand ideas, and we’re bound to lose some games, maybe even more games, with the wild approach. It’s not that the sideline is unsound, but because even with good prep we’re going for situations of increased volatility, and (by the standard of perfection) we’re not very good at chess (again, spend some time with Stockfish if you think otherwise - even if you’re a titled player). We’ll slip on our banana peels every now and then. But with good prep, we know where those peels are, and our opponent will have to discover them on his own; not an easy task. So we might lose more games than we otherwise would when hiding behind mainstream theory (especially in solid, dull-ish lines) and maybe our opponents will occasionally get easier draws. On balance, though, we’re likely to get more victories, and they will often be speedy wins - always important for those of us playing in Swiss events with multiple rounds a day.
Just to clarify: When speaking of “off-brand” lines I’m not talking about the Grob (1.g4??-+), the Matrix (1.e4 e5 2.Qh5), or the Elephant Gambit (1.e4 e5 2.Nf3 d5), or silliness like the Bong Cloud (1.e4, 2.Ke2) - though in some contexts they might work as well. The sort of line I have in mind is perfectly respectable, objectively speaking, but will involve moves that are outside of your engine’s top three or four moves at one or more points. It’s not that hard to find such moves, and sometimes they radically change the nature of the play from what one would expect from the engine’s top moves.
There are players who specialize in this sort of thing: Magnus Carlsen, of course, but plenty of others. The greatest exemplar of this, I think, is Daniil Dubov, who comes up with fantastic opening ideas on a regular basis, which often lead him to smashing victories even against players in the absolute elite, particularly in rapid and blitz. It’s something all of us should try to incorporate in our repertoire, however, as time and energy permit. It will even help you to have a fighting chance against a stronger opponent.