VILTROX EF-Z2 0.71x Speed Booster is designed to compatible with Canon EF-mount lenses to be used on Nikon Z-mount mirrorless cameras. With the built-in optical correction group. Focal length is multiplied by 0.71 based on the original (equivalent focal length = focal length x 0.71 x APS-C conversion factor), makes the effective focal length smaller and the shooting range wider.
Also the speed booster can increase one stop aperture of lens to blur the background and better highlight the theme.