At the highest level, modern rugby is about who can expend the least energy to score as many points as possible.

The old way of thinking — that defence is more exhausting than attack — only really applies in very specific circumstances these days. You hear all the time during games about how one team or another has racked up a massive number of tackles in a half, and the worry is whether they can maintain that for the rest of the game. SPOILER: They almost always can.

Just at a practical level, it makes sense when you think about it. A long attacking sequence involves a carry, at least one ruck support player, but usually two or more, supporting the carry, a scrumhalf buzzing from ruck to ruck, another pod of forwards getting into position for the same thing, and midfielders and back three players running exhausting loop lines across the pitch. Drag this out to five, six or seven phases multiple times a game, multiple times a season, and you can see how attritional it is.

On the defensive side, however, it’s far from easy, but the energy expenditure is significantly less. Only two or three defenders are directly involved with the tackle and subsequent ruck, with the rest of the team fanning out, making reads, and jogging into position. That energy expenditure goes up if you play a particularly aggressive blitz defence, but, for the most part, if an attacking team holds the ball for five phases outside of your 22, they will expend more energy than the defensive team.

The only place where the energy usage is equal — or even in the attacking team’s favour — is when the ball is on the 5m line.

Every team at the elite level is well aware of the tradeoff. If you play a high-possession brand of rugby, you can overload defensive teams with cognitive load. At a basic level, you’re asking if they defend without an error for multiple sequences, or almost more importantly, without conceding a defensive penalty for six or seven phases.

When you have the ball, the numbers are quietly on your side. Every public study that breaks penalties down by attack vs defence finds the same thing: defending teams concede more. A study of Super Rugby across multiple seasons found more penalties awarded against the defending team in every single year analysed. It’s not a marginal split — to be clear — it’s the trend, full stop.

That makes sense once you look at the menu of offences. Most of the bread-and-butter penalties are defensive by nature: not rolling away, side entry at the ruck, hands in, offside at the breakdown, and high tackles. The attacking team’s list is shorter and more avoidable — holding on, obstruction, and the odd crossing call.

So when a side commits to a high-possession game, they’re not just hoping the defence misses a tackle. You don’t really have full control over that. They’re banking on a much more reliable proposition: that somewhere in the next five or six phases, a defender’s hand will end up where it shouldn’t, or a tackler won’t quite get back to his feet in time, or that they can be pinned into the ruck for a penalty that will march you down the field.

Glasgow are a really good example of this play-style, with Ulster a close second. This often comes with a cost, though, with attritional injuries at the macro level, and large swings in games at the micro level.

The other extreme is a broadly off-ball, transition style of rugby — France are a great example of this — that uses a lot of kicking to decide where the next sequence should be played and either defending those sequences where the ball lands, or trading kicks to bring their transition game into action.

In the modern game, the quality of your attacking and defensive transition game, specifically, dictates winners and losers as much as any other traditional metric.

This is where energy expenditure starts to even itself out. If defending expends less energy than settled phase play, then transition balances the scales. Every kick you launch, be it long or short, has to be chased by everyone — your wingers, midfielders and flankers have to cover the most ground, with your front five coming up after plugging the gaps along the line of the kick. When the ball lands, you should be in a flat line across the field as much as possible. Kick longer, and you expend more energy to chase it, while also demanding a little more of the opposition — you run to the ruck point, they have to go beyond it. Kick shorter, and you increase your chance of retaining the kick, save your own energy, have a little more control over the resulting transition, but the ball is closer to your try-line than the alternative, and you can be easily kicked back into your 22, where you might well have to exit to touch under pressure.

Whatever happens, the team receiving the kick now have to find space somewhere, or reset to kick in return.

Here’s a great example from the recent UBB vs Bath European Cup semi-final, where you’ll see both sides look to manage their energy, while also probing for transition opportunities.

And so it becomes a game of mistakes, or half-mistakes, where a kick too long, or too short, or a badly timed chase or lost duel becomes the gap where magic can happen.

For most sides, though, the area where you choose to expend energy becomes hugely important. Offensively, that means most teams will only go through multiple phases inside the opposition’s 10m line, or at the set piece, where the current balance of the game means that any penalties or turnovers you can win at a scrum or lineout/lineout maul have never been more valuable.

All of which brings us back to where we started: this is an accounting problem. Every team has a finite amount of energy to spend across 80 minutes, and the sides that win consistently are the ones who know exactly which line items are worth the spend.

Phase play inside the opposition 22? Worth it. The defence is compressed, the pitch is short, and every ruck is a chance to either score or draw the penalty that lets you tap and go five metres out. Phase play inside your own half? Seldom worth it. You’re asking your forwards to carry into a defensive set defence of sorts, your scrumhalf to sprint between rucks, and your back three to run support lines, all for territory you could have gained with a single kick. Worse, every ruck is a chance for you to give away the penalty or a turnover, and now you’re defending on your own 22, having spent the energy to get there.

Transition is where the accounting gets interesting, because both sides are spending at the same rate. The kick goes up, everyone runs, and whoever wins the duel — or whoever’s chase line is half a metre flatter — gets to dictate where the next exchange happens. It’s the closest thing modern rugby has to a weighted coin flip, which is exactly why the best sides have spent the last few seasons obsessing over it.

Defence, set piece, transition, and selective phase play in the right zones. That’s the modern energy budget. The teams who get it right aren’t necessarily the most talented or the fittest — they’re the ones who’ve worked out that expending too much energy for 80 minutes is a mug’s game, and that the real skill is knowing when not to.