It's just a time bomb, the signs should explain it.
When water reaches certain level, BOOM!
If no water is detected (aka, someone has tampered with the bomb or the mechanism didn't work) then a heat timer is set off, it takes a second for the heat timer to open the heat valve, which releases plasma, heating the diamond wire, making the C4 explode.
The heat timer will turn off if water is detected again.
Yeah, it takes a second for the NOT gate to begin work, its a flaw with the NOT gate and a sort of feature to stop the heat valve opening if you were to, say, shake the bomb.
IRL, most bombs take hours, if you want to speed it up, add more water yourself.
super_mario
Yeah, thanks for crediting me, but the idea was that people could also see how it worked, still, I may try making a smaller timer and a smaller bomb later.