Croc Coulter works with an apprentice to revive the lost art of tātatau, traditional tattooing done with sticks and bone. So that's how they made such straight lines in geometric tattoos!
From the Loading Docs site:
Croc Coulter, a heavily tattooed Englishman living with cystic fibrosis in Rarotonga, teaches his new apprentice the traditional Polynesian art of Tātatau. As with all indigenous art forms, the relationship between the master and apprentice is an important one. In order for the traditional practice to continue, the knowledge must be passed down.
It's part of an interesting series called 'Aka'ōu.