The Quirk allows the user to grow long, sharp claws from their fingers, spines on their back, and a shield visor on their face, to help them dig through the Earth, just like a mole. The spines on the back allowing the user to much more easily slide underground, as well as blend into their environment, and the shield used to protect their eyes.
The claws can also be utilized to tear through many obstacles, like forestry, enemies, or even buildings, as seen through Gigantomachia, who, thanks to his enhanced strength and size, is able to use the claws to tear through and clear out all obstacles with ease.
All For One gave Gigantomachia the Quirk to allow him to lie dormant, hiding deep underground in the mountains, shortly before being crippled by All Might. He would continue to use this as his way of resting, for the few hours he does, as well as remaining on standby waiting for an order from one of his masters.
He utilizes the transformative abilities of the Quirk during the Paranormal Liberation War. After the U.A. students part of the Villa Raid Team successfully insert the sedative into his mouth, and the remaining Pro Heroes prepare to attack, he transforms his hands into the giant claws, which he uses to completely decimate all the incoming foes.
The claws also prove to be incredibly useful at terraforming the buildings and everything in his path as he easily rampages through all the cities before eventually making it to Tomura Shigaraki's location.
It appears that Gigantomachia can choose which parts of the Mole Quirk to transform into, as he keeps the rock-like spines on his back activated at all times, while deciding to turn on and off the claws and visor.