Maintaining pressure on Pressure Cooker
In many electric pressure cookers, you have (at least) two options: "High Pressure" or "Low Pressure".  This effectively controls the heat at which your food is cooked.  That's due to how a pressure cooker operates.  It cooks by raising the temperature of water to create steam, and then continues creating steam, which raises the pressure inside the vessel, thus raising the temperature at which water boils significantly.  (See this article for some information about the science behind pressure cooking).

Now, in a stovetop pressure cooker, if you follow the instructions you'll find out that exactly what you're asking does happen: you need to reduce the heat on the stovetop once your cooker has come to the pressure you're aiming for in order to stay at that pressure.  Your pressure cooker's manual should cover how to tell this; exactly what that heat is will depend on your stove's capacity.  Your pressure cooker should have some variety of pressure gauge; most will have at least a ball that sits in the steam exhaust vent which, when it is at pressure, sits at the top of the vent.  Once that's at the top of the vent, you are at low pressure; then you lower the heat as low as possible such that the ball stays at the top of the vent, and that's cooking at low pressure.  If you need to cook at higher pressure, which will cook faster (and is necessary for some foods), you have the heat up some from that (and make sure to include enough water to produce enough steam).  Here is an article that covers these basics.

For beans, you don't have to cook them very long - a couple of minutes - and you should use high pressure (whatever the 'high' pressure is for your cooker).  Assuming you pre-soak, one or two minutes is long enough to cook them (at high pressure) - so you won't really need to lower the heat for very long, and in my experience it's usually hard to be there at exactly the right time to lower it (if you have a couple of toddlers underfoot in particular); don't worry if you forget or miss it, as long as you stop the cooking at the right time; overcooked beans are no fun.  The higher heat isn't going to burn them (as long as you included plenty of water), it's just going to waste energy, as your pressure cooker will just give off the excess steam.  (This is based on normal ranges - if you have something unusually heat-additive, including an induction burner, I don't know if it's reasonably possible to do something dangerous in one or two minutes.)

You're basically right, though, that 'simmering' is roughly what you're doing when you are using the pressure cooker; reducing the boil from high rolling boil (which produces lots of steam) to a lower boil (which produces just enough steam to offset the loss from the exhaust port).  It's just a higher heat setting than normal simmering, since pressure is increasing the temperature at which water boils.

Your manual should cover how to do this; I'm not sure which product you exactly have, but look here for one example.