Original: If the Checkuser tool was used to determine that sockpuppetry had occurred, only another administrator with access to Checkuser can overturn the block as only they have access to the evidence on which it was based.
Simplified: If the meta:Checkuser tool was used to determine that sockpuppetry had occurred, only another administrator with access to Checkuser can overturn the block as only they have access to the evidence on which it was based.