Not sure how well I can explain this. but I'll try. You get this with front fastening and some SS (in my experience) nappies when the 'win g' or tab with the bottom poppers drops underneath the top fastening poppers. Best eg are Ittis. I get wing droop when Max has wet the nappy alot and the tabs have slipped down and actually poke out of near the top of the thighs rather than staying tucked in properly.
I don't think I have explained that very well and could really do with a pic to illustrate but hopefully it will give you an idea