Modding mutation questions

I’m just starting out and have little grasp on how coding works. How does the code for mutations work? I see that some have visible stat changes but others that have clear effects in game have no details at all. for instance quills.
“type” : “mutation”,
“id” : “QUILLS”,
“name” : “Quills”,
“points” : 2,
“visibility” : 3,
“ugliness” : 3,
“description” : “Your body is covered with large quills. Whenever an unarmed opponent strikes a part of your body that is not covered by clothing, they will receive significant damage.”,
“prereqs” : [“SPINES”]
where is the part that causes the actual damage return effect?