What is the "mode" button used for the megadrive/genesis six-button controller?

[Chaotic Forces]

I've no flippin' idea, any of yis know?

[Bloodreign]

For games like Ms. Pac Man and Forgotten Worlds that only requires 3 buttons, you hold the mode button as you power on the unit (thereby turning the pad into 3 buttons only). For Ms. Pac Man if you don't do this, your control over Ms. Pac Man won't be there, she'll move with a mind of her own, for Forgotten Worlds if this is not done, right as you start the game it will auto game over.