Poke 2 & 3, Max 3 use Qualcomm processors which have (like all Qualcomm) "Emergency Download Mode", EDL.
This is a ROM-based bootloader which does not require anything in flash to run.
If you screw things up really bad your device may go to EDL by itself.
If things are working fine you could go "adb reboot edl" or in a shell "reboot edl".
If things are halfway screwed up you may not be able to reach EDL programatically.
That is why many Qualcomm devices put in special test points on the circuit board to force it into EDL mode.
You short these test points together while you are doing a power button reset.
Ok, most people will
never have anything to do with EDL mode.
You also will need special software to talk EDL also:
https://github.com/bkerler/edl
But what if you're the type that always gets into trouble and you hate opening the back of your device all the time?
Well, I put a little magnetic reed switch across the test points so that I won't have to open my device (Poke 3) ever.
It's specifically insensitive so that the magnetic book cover can't activate it.
You need to place a big magnet on the back of the cover and press the power button.
As a special offer and if you really think you will use this, PM me and I'll drop a reed switch in the mail for you.
I really can't imagine this applies to too many people; I only have a couple.
The white connector? Oh, that's the console UART.
It runs on 1.8V logic level, 115200-8-n-1
You'll need a level converter probably.