Fucking Ridiculous Eating Device

The upper part of the frame including the cylinder and barrel recoil backward after firing. As the upper frame moves backward, the hammer is re-cocked. 

[Source]As the upper frame assembly returns forward, the cylinder rotates the next chamber into firing position.

The Mateba Unica 6 is not the first to use this kind of system.The Webley-Fosbery (1900) is an earlier example that gained a lot of attention due to the unique nature of its function. They are easily identified by the zig-zag grooves that are used to rotate the cylinder as the upper assembly moves back and forth under recoil.

[Source]While unrelated, it is interesting to note that the famed Pancor Jackhammer shotgun prototype incorporated a similar cylinder groove cam system its its design, that can be seen here: