First year trying these things mostly with sluggos. I noticed they have two eyelets. One at the tip and one at the base. What's that for? I have no idea other that maybe more action if you clip in at the base vs. tip.
In a lot of the old tin squid jigs for rigged eels there is a second eyelet. This is usually used to tie the 2nd or tail hook to. You (like the McKenna rigged Sluggo) run the line from the tail hook through the eel (or rubber) and tie it off to the 2nd ring.You can also tie your main line to the 2nd ring if you want it to run deeper.It will "dig in" and run lower this way.