When casting jigs .... set the hook on ANY feeling of a "thump", line jump or sideways movement, or in the case of sudden slack line (when you know the jig cannot possibly be on the bottom). And yes ... you set the hook IMMEDIATELY upon seeing or feeling any of these indications.
And while you can usually tell when Bluegill are hitting your jig, by the rat-a-tat-tat cadence of the hits, or a really hard yank & run ... it's not always Bluegill. Case in point : I was casting a jig at a bridge pillar and slowly reeling it back. This was after I had already fished the pillar from close up, and caught some fish, but the bite had slacked off. I assumed there were more fish in the area, but that they may have moved away from the pillar due to the commotion. I kept getting these "electric" vibrations through the rod, that felt almost like dragging the jig on a sidewalk. I first thought it may have been small Bluegill nipping at the tail, or possibly the Herring Shad that are known to be in those waters & will eat minnows and hit jigs. I was about to give up on the area & go somewhere else, but as I was contemplating that, I had that same "vibration" occur as I was retrieving my jig. I got a little "miffed" and set the hook in anger/frustration. To my surprise, I had hooked into a Crappie of about 1.5lbs I continued setting the hook when this vibration feeling occurred, and landed several more Crappie of about the same size. I told my fishing partners about the "feeling" and to set the hook on any retrieve that didn't come back smoothly ... and they, too, started nailing fish.
what those fish were doing, or why, but I sure don't hesitate to set the hook on ANY strange feeling or line movement, anymore !! I don't connect with fish on all such occasions ... but, I do think my hookup ratio has increased by me setting the hook immediately upon seeing or feeling anything I perceive as a hit, regardless of how subtle or strange it might be.
... cp