The polenta mixture should be very runny, like the consistency of soup. Put the lid on, and immediately lower the heat to medium-low, depending on the strength of your hot-plates. If you are using gas, turn it down to the lowest flame you can.
Let it cook for 30 minutes. Keep an eye on the pressure. If the safety valve drops down, turn the heat up a little to make it pop up again, then lower the heat again. You don't need to hear the mixture bubbling inside. In fact it's better if you don't. If it is boiling strongly, you will have burnt polenta on the bottom of the pot.
After 30 minutes, run the pressure cooker under cold water to decrease the pressure.
Take the lid off and stir the polenta. It's now ready!!
Note: if the polenta is too thick when it has finished cooking, don't panic. Just add hot water and stir it until it becomes creamy and runnier.
This recipe is for 2 - 4 people, depending on how you are going to use the polenta.