>What on earth do these chips have that is necessary for (ARM) Windows 10, that their previous chips don't?
x86 emulation? So you can run Windows' massive legacy software base.
MS was stuffed by Intel's exit from the mobile market. Without their legacy software MS has little to offer.
Intel's problem is that they don't want to do cheap. Why create a cheap Atom system when an i5 is so much more lucrative? MS' problem is that most users don't want underpowered Windows and those that do, don't want to pay full-fat licensing. It is a lethal combination for any mobile Windows solution. Essentially MS will have to give mobile licenses away as a bundle with something else. Hence... Cloud. With cloud the low-power cpu issue goes away and you can tie licenses to named users, preventing mobile licenses from being used simultaneously with desktop ones by different users.