BOP, or Balance of Performance, is a way to even out performance differences across different manufacturers. There are various reasons to do that, but the main one is to make the field competitive and lower development costs.
What gives?
Before we start explaining what BOP is, you need to acknowledge one key element about racing; money. Racing is a very expensive endeavor. No matter at what level you want to compete in, there's a hefty price tag to consider. That makes it difficult for racing teams and manufacturers to put cars or bikes on the grid.
When a car is (overly) dominant in a series, that's great for that single manufacturer, but the others suffer. Not just in ego, but in financial deals with sponsors, drivers, and other sources of income. That's why you often see teams appear and disappear; they lack funding.
For the organizers, it's thus important to make the top spot(s) accessible for a wide array of manufacturers. Yes, drivers and teams can still make the difference, but it stops the grid from turning into a single manufacturer class. Those of you that watch MotoGP know what I'm talking about. Ducati is incredibly dominant, meaning that basically every rider on the grid wants one. That's good for Ducati, and it doesn't even affect the racing itself too much. However, other manufacturers now have to spend millions in development in hopes to catch up. Not only that, but sponsors are losing interest in these teams as they're not getting enough publicity or TV-time.
So what is it exactly?
Well, BOP is measures taken to find parity between the cars or bikes on the grid. That means the fastest ones get slowed down and the slower ones are made faster. There are various ways to do this.
Weight is one of the main changes. The total mass of a vehicle is a deciding element in handling and overall speed. By adding weight to faster cars, they get slowed down. It's not necessarily slower on the straights, but the added weight will upset the car's balance under braking and influence the car's cornering capabilities.
Rev limits are frequently tuned. The maximum rev limit of a car has an impact on the horsepower output and efficiency. By reducing the rev limits, the overall power output can be reduced just for the car to lose its advantage on straights or out of corners.
But these are just a few of the modifications made. Additionally, the organizers also play around with air restrictor size, boost ratios, rear wing levels, and fuel capacity of the cars.
Is BOP good or bad?
It depends on your perspective on racing. To me, personally, I think it's a good thing under certain circumstances.
Production racing
If we look at GT3 racing, this class was intended to be a 'low' budget, accessible class. BOP supports that concept as even the slower production car can still compete at the front. For classes that are based on production models, BOP makes a lot of sense. It would otherwise force manufacturers to spend more millions into developing faster cars that could compromise their handling on the road. Not only that, but in no time, you'd have a Porsche or Ferrari cup.
Prototype racing
In these kinds of classes, BOP should be limited. There already are strict regulations about testing and development in place. Think of F1 or MotoGP. These regulations are in part designed to reduce costs, but they often fail to do this effectively. But in these classes, we also see technical innovations that motorsport (and the automotive industry in general) need. Rule changes are far more effective to push manufacturers to rethink their concepts and get creative. Something you don't necessarily need for production vehicles that cost a fraction of the price.
Conclusion
BOP brings closer racing in certain categories. It's always a downside when you're on the receiving end of getting a BOP change to slow your car down. However, considering that it's meant to bring more teams, drivers, and attention to racing, it's a good thing. Formula 1 or MotoGP should steer clear for now. Which doesn't mean they shouldn't update their regulations to bring other manufacturers to the front.
Hate it or love it, you have to deal with it. And in any regard… that's racing.