it seems a bug from stock N900 which could trigger only with non-stock bootware. Or a bug in multiboot/bootmenu only. Why the approach helped? N900 were probably in locked state which couldn't be left normally (so-called reboot loop for example) and pressing the button changed hardware state which you were lucky enough to lock-out. It's common that devices are hardware-optimised to work faster and with less transistors to achieve a result, which renders undefined behaviour if happened something which shouldn't never happen (for example CPU got invalid command, or unstable because of eg overclocking chip changes a value of register to one which would never be there if it were stable). Of course many companies left a bit of optimisation just for a sake to device won't be bricked without physical damage for hardware reasons (burning some transistors because of overheat caused for example by overclocking is physical damage, wrong memory/register values because of unstable work of device aren't) but it's wasting at least thousands of dollars for fixing a problem which would happen to only 3 people which will use the device - so mostly companies don't mind it. As you saw most of such problems could be resolved in home way by just doing some voodoo-like tricks on the device like pressing Power button exactly in some second of bootprocess.