The standard one is probably a bleeder type MBC. A bleeder type will bleed off the boost signalt to the w/g or actuator in order to hold the w/g closed longer. A ball and spring type uses a spring to hold a ball bearing against the opening in the MBC until it reaches a boost pressure high enough to compress the spring. Then the ball is pushed out of the opening and the boost signal reaches the w/g or actuator and opens the w/g. Ball/spring type MBC's are generally better and less prone to spiking than bleeder types.