The answer for this question varies from trolling lure to fly. The IGFA’s rules state that for skirted trolling lures, “the trailing hook may not extend more than a hook’s length beyond the skirt of the lure”. This means that as long as some part of the hook is within the skirts – perhaps only the eye – it complies. However, the IGFA’s rules for fly fishing are different and state that: “the second hook in any tandem-hook fly must not extend beyond the wing material”.
Therefore, when using a trolling lure, it is legal for your trailing hook to be only partially enclosed within the skirt. However, when fly fishing, the trailing hook must be completely enclosed in the ‘wing’ material of the fly.