Fish in a Bag

I am looking for tips on how best to cook fish in transparent silicone ovenproof foil. This foil looks like cling-film but is not cling-film, it can withstand temperatures of up to 200°C / 390°F. In principle you just place on the foil the fish, fillets or whole, with seasoning and a little liquid, oil, lemon etc, then close it like a parcel in the shape of a boat and firmly secure the edges with silicone ties, supplied with the foil. Some say when the parcel puffs up with air the fish is done, but I am not sure about this. Has anyone had experience with this type of cooking?