In the original Spectrum, there was no reset button. Nor was there a power switch. So if your machine code got stuck, you'd remove the power plug from the jack and put it back in, to get that power-on self test black screen and then a BASIC prompt.

And, eventually, the jack would wear out for those of us who wound up with more bugs than was good for the power jack. Some people actually had to replace the power jack because of this. You can still get those as spare parts, btw. http://www.dataserve-retro.co.uk/contents/en-uk/d7.html

The Z80 has a non-maskable interrupt that jumps to a fixed location in the ROM, so anyone handy with a soldering iron could have wired up a little push-button to the NMI pin to trigger it ... in the Specrum ROM this was supposed to then read an address from a fixed area in the system variables area of RAM and jump to it, except that due to a bug you could only jump to 0, ie a full reset. This always seemed a terrible shame.

Yes, I recall looking at that code in the ROM disassembly and trying to figure out that what the heck is this supposed to mean.

Yes... it was friendly to budding hackers like myself in many ways. Pretty clearly intended to be a computer, not what another commenter calls "better machine".