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.