chkButtons() scans each button pinsBut[] and compares the current value to the previous value, buttonState to determine if the state of the button has changed: either pressed, going from HIGH to LOW, or released, from LOW to HIGH. it does nothing for release.
it reports the first button recognized as pressed (but the assumption is only one button is being pressed at a time). after being pressed, and potentially while still being pressed, a subsequent call to chkButtons() no longer reports a press.
a button press is a single event that needs to be processed when recognized