The attack involves Rilldo blowing a green energy-comprised breath that turns whatever it strikes into metal. Rilldo can make the beam spread to follow someone, as seen in the Super 17 Saga.
Once he reaches his liquid metal form and after hitting things with the beam, he shows the ability to control those hit with the beam and manipulate them as if they were water, once throwing a tide of liquid metal at Goku.
General Rilldo first uses this move against Goku, Pan and Trunks. He successfully strikes Trunks and manages to hit Goku with the attack later (unbeknownst to Rilldo, this is all part of Trunks' and Giru's plan). In the Super 17 Saga, Rilldo uses it to change Gohan's arm and leg into metal after he escaped from Hell, with Gohan unable to move his affected limbs. When he was defeated again and sent back to Hell, the effects on Gohan were lifted thanks to Giru.
Interestingly, it seems that General Rilldo's metal may inhibit ones natural ability to use ki. When Goku is hit by the beam, and then turned to metal, he loses his Super Saiyan form (despite the fact that he was not knocked out of it.) It's also possible he improved the technique, while in hell. When he uses it on Gohan, he can cover single limbs, whereas on M2 anyone hit had their entire body encased in metal.