Let Them Come (UP)

When you fill the Combo Meter (by killing enemies in a row), you'll get a chance to choose one of three power-ups. When you select it, a meter will fill. Press A at the top of the meter (without going over) to get a "Perfect", which makes the power-up the most effective or last the longest.

This is easiest the first time you get it, which goes into a bit of slo-mo to explain the system.