i don't understand how to keep fingers flat or lift them, don't you have to lift them to press the key? do you mean your wrists up or down? i pick my wrists up for jacks and put them back down for streams
Not sure what you mean by fingers being flat, but going off of the previous comment I play entirely wrist down except when I vibro(which is almost never)
I usually tend to flatten them with heavy one-hand trilling and pointed up more for chordjack heavy patterns (for both wrist up and wrist down hand positions).