The [[ABBA switch]] is a common hardware modification that allows users to swap between Drive A and Drive B.
On Amstrad CPC, some software games and disk utilities other software were designed to run specifically from Drive A, creating a problem when the disk you needed was in Drive B.
The ABBA switch resolved this issue by allowing you to "swap" the designation of the two drives. When the switch is activated, Drive A becomes Drive B, and vice versa, effectively swapping their roles.