Not sure about the 3DS ntrboot flashcarts exactly, but a "time bomb" on flashcarts in general refers to an underhanded way sellers try to get you to replace your flashcart every so often (not actually that often, but still). There's either a sort of internal clock that counts down to 0 and when that happens, the flashcart is completely unusable or, more commonly, a function that checks the date of the system you are using the flashcart with and if it's past a certain date (eg: 3 years after day of production or after the day it's first used), the same thing as the aforementioned case happens. If the time bomb has been triggered, you can't use ntrboot, so you have to get a new flashcart.