The shift functions are handled by keyboard encoders and are not a part of mame. What kind of keyboard encoder are you using, is it a jamma cabinet?? You can get a "shift" effect in mame however. Just go into the setup menu(tab) and pick a control you want to customize. Take "pause or p" for example. Edit the pause input to the keys that you want to use. For example press "1 & Ctrl" at the same time. The pause command now will only work when player 1 start and p1 button1 are pressed at the same time. I hope this answers your question.