A trapped blister is almost the same as a traditional blister pack but the plastic part that sticks to the cardboard has another piece of cardboard in front of it that fits around the shape of the blister. The plastic is then trapped between two pieces of cardboard. The top piece of card is die-cut to fit the blister where the product sits.
As this type of blister packaging uses no heat-sealing methods, it’s cheaper to produce than those methods that do.
This makes it more tamper-proof as it becomes obvious if the front cardboard has been ripped. It also looks better as it covers the plastic.