Many ARM SOCs use a "BIG.little"[0] CPU technique where they can swap from a high-power dual- or quad-core section of the CPU to a low-power quad-core section.
> Inside the iPhone 8 and iPhone 8 Plus, there's a new six-core A11 Bionic chip... There are two performance cores that are 25 percent faster than the A10 Fusion chip in the iPhone 7, and four efficiency cores that are 70 percent faster.[1]
They've added 2 low-power cores, and made the cores faster than the previous version. Normally, only one set of cores could operate at once.
> The A11 features an Apple-designed 64-bit ARMv8-A six-core CPU, with two high-performance cores at 2.39 GHz, called Monsoon, and four energy-efficient cores, called Mistral.[1][6][4] The A11 uses a new second-generation performance controller, which permits the A11 to use all six cores simultaneously,[8] unlike its predecessor the A10.[2]
Here, the 2 high-power and 4 low-power cores can all operate at once, alongside the 3-core GPU - 2 cores of which can assist ML tasks too.
> There's a new Apple-designed 3-core GPU that's 30 percent faster than the previous-generation GPU, and two of the cores, the Neural Engine, make machine learning tasks faster than ever.[1,ibid]
Not sure how much power it consumes in that mode, but it's reported to use half the energy of the A10.
> In terms of graphics performance, the GPU is said to be 30% faster than last year’s, while consuming half the power when working at the same rate as the A10.[3]
[0] https://en.wikipedia.org/wiki/ARM_big.LITTLE
[1] https://www.macrumors.com/roundup/iphone-8/
[2] https://en.wikipedia.org/wiki/Apple_A11
[3] http://www.trustedreviews.com/news/apple-a11-processor-specs...