it hooks SecTrustEvaluate function which evaluates the trust manager object and check the rule from object and check the certificate against the rule. If it is value it return 0 .
Here is how SecTrustEvaluate function
OSStatus SecTrustEvaluate(SecTrustRef trust, SecTrustResultType *result);
the script hook the function and edit return to 0.
frida -U -l sslKill.js Gadget