This is a sidescrolling shooter but, unlike most shooters, you do not pick up power-ups. Instead, you increase power by killing enemies. You can only change weapons when you complete a level or die.
You cannot actually change weapons during a level unless you die. If you press the second button then your current weapon will change its behaviour. You can use a single large beam, two rotating beams and two others (these are difficult to describe). There are also four weapon types: vulcan, laser, atomic and homing.