The iPhone 15 Pro and Pro Max will incorporate a new ultra-low power coprocessor inside the expected A17 Bionic chip. The power-sipping chip will allow features like the new solid-state volume and power buttons to remain usable even when the device is turned off or the battery has been depleted, according to a source that explained details on the MacRumors forums.

According to the source, the new coprocessor will replace the current “super low energy mode” that runs off the main SoC. The current mode manages a number of certain features even when an iPhone is not turned on. For example, an iPhone running iOS 15 and later can still update its Find My location after it has been powered off or for up to 24 hours if its battery has ran out. The mode also enables Apple Pay Express Mode for up to five hours after the battery has ran empty.

The source adds that the coprocessor will take over those tasks, and more significantly, will be able to “immediately sense capacitive button presses, holds, and even detect their own version of 3D Touch”. The coprocessor will manage interactions with the new volume and power buttons, in addition to a new “action button” in lieu of the mechanical ringer switch. The coprocessor will reportedly still be able to sense button inputs while the phone is powered down or has ran out of battery.

The source states that there are two Taptic Engine modes currently being tested: one that triggers haptic feedback even when the device is turned off, and another that limits haptics only when the device is on. However, a mode with haptics enabled with a depleted battery is not being considered.

Apple is internally debating two modes for the unified volume button, said the source. The first mode would change volume faster depending on how much pressure is applied to the button akin to 3D Touch. The second mode would incorporate a swiping mechanism by which swiping up and down on the unified button adjusts volume. The source is currently unsure which implementation Apple would settle on. However, both modes are ready to be enabled by software “at any point as the 3D-touch buttons are definite.”

In a follow-up comment, the source further claims that the new button replacing the mute switch is indeed internally referred to as the “action” button. Like the Apple Watch Ultra, the new button is designated to be remappable to perform customizable functions.

The new 3D Touch-alike buttons are expected to be exclusive to the iPhone 15 Pro and 15 Pro Max. The base iPhone 15 and 15 Plus are expected to stick to standard mechanical buttons for at least another year.