@Psybermagi has to answer that one.
I am not sure where that description is drawn from. That's not how it works "by the book".
In TD2e, you can't explicitly choose Unarmed as a weapon group proficiency. You can only get it through the Martial Arts trait. You can't have any Mastery with unarmed in TD2e. Instead you can gain Martial Arts Disciplines as a prestige trait. (And they are cool! They are worth getting if you take Martial Arts first.)
However, other games in the Tiny D6 family do change things up some. In Tiny Supers, you can take Unarmed as a weapon group proficiency and can get Mastery, too. (The mastery there is typically listed as "Haymaker", which is just their big punch.) Maybe that's the idea of where the text you mention is taken from. In that case, the mastery would be "punch", "haymaker", "roundhouse kick", "karate chop", or something like that.
Do note that even if you can get Unarmed proficiency without Martial Artist, the trait is still worthwhile. This is because if you use Martial Artist, you do get that other proficiency, you get that one reroll a day, and you unlock the ability to take those Martial Arts Disciplines. And, you can probably still get Mastery in "karate chop" later, too, after gaining experience.