Why are Mercedes struggling to understand George Russell's straight-line deficit? They must have all the data on his battery deployment, ICE output, e-motor power, plus how much drag his aero and car set-up, such as rake angle, generates - Sean

At the Belgian Grand Prix, Mercedes experienced unexplained behaviour from their power unit.

Russell suffered for this, by having a mystery lack of energy over a lap in qualifying. It cost him some but not all of his deficit to team-mate Kimi Antonelli. Mercedes team principal Toto Wolff put the loss from the engine at about 0.2-0.25 seconds.

A pretty much identical thing happened at McLaren to Oscar Piastri in relation to Lando Norris.

Both drivers also experienced a shortfall of energy on the first lap that contributed both to Russell's incident with Hamilton and Antonelli being passed by Max Verstappen on the approach to Eau Rouge.

As Wolff said: "All Mercedes engines had a lack of energy out of Turn One so that wasn't only George. It affected him more than Kimi but also Kimi had that issue to a certain degree."

The one in the race was, Wolff said, "a new issue".

Mercedes said they did not understand why either situation had happened.

The fundamental issue is that the latest F1 hybrid engines are highly complex, as are the rules by which they have to be operated, and the manufacturers and teams are still learning about them.

One of the impacts of this is that small nuances of driving can have a disproportionately large impact on energy availability. It's one of the reasons why the drivers do not like these engines.

McLaren team principal Andrea Stella, among the best technical communicators in the sport, has talked about this extensively this season.

At the weekend in Spa he said: "To operate these power units, you don't operate them in an open-loop way, whereby I would be able to have an off-line simulation and say, 'Oh, that's how the electrical power will be deployed and it will always be identical to itself'.

"There's a large component of the controller that operates the power unit that happens with live calculations and the power unit forward-thinks, forward-calculates what it has to do. There's a lot that happens as the car goes.

"This is why it's not so easy to understand what kind of calculations have been made as the car was going.

"I'm sure we will understand the reason, but at the same time you need to be able to anticipate and make sure that the operation of the power unit unfolds as expected.

"Because this is not only for the influence it has in the power-limited sections, [and] in the straights, but also because it affects the braking points.

"Because if you have an additional amount of harvest before braking, then you are approaching the braking 10km/h slower and your braking point changes.

"This is quite difficult to master for the drivers, and it's one of the reasons why they talk about the difficulties of exploiting the power unit from a driving point of view.

"It's not only to get the most out of the power unit, but also because the variations of the power unit affect the references as you approach a corner.

"But the variations that are significantly relevant are so small that they may happen from one lap to the other, even if you have learned from the previous laps."

This complexity also explains why McLaren have been frustrated this season by some aspects of their relationship with Mercedes, particularly in terms of how much information they have been given about the operation of the engine.

As Stella said: "It's only now, and we are mid-season, that we are starting to have the required tools in order to be able to anticipate, optimise, and study these sensitivities."