The gray meter that appears at the bottom of your screen indicates that you can charge up the weapon you have selected in your arsenal. Hold L to charge for as long as desired and release it to unleash the weapon. However, when the bar fills completely red you must release L or the weapon will backfire and not go anywhere. For example, Axel's EMP special move will have an increasingly larger blast radius the longer you hold L, but will be useless if you over-charge. Note: The gray meter only appears after you've received the sixth "Special Weapons" upgrade by defeating enemies.