Sounds like there is no Fail safe mechanism and no shut off either! From what you said it does sound a bit low-tech and it does seem to be a better idea to save up and buy a kit with all the accessories and controllers. It's not as easy as spraying N2O gas down the intake otherwise all the expensive controllers and kits wouldn't exist!
The long and short of it however is that when it is sprayed in the intake it will eventually reach the cylinders, breaking down in the compression and temperature extremes forming a mix of Oxygen, Nitrates and other compounds when it combines with fuel. The Nitrogen aids to prevent detonation / knocking but the power comes from the Oxygen. You will need some sort of ECU / Fuel map if you want to run high power nitrous as the extra Oxygen makes the ECU request extra fuel: Hence a bigger bang and more power from the charge! If the ECU hits a limit it will run lean.....