According to Boyle's law, while pressure of water increases as we go deeper volume of gases decrease. This way, when a diver goes deeper into water, air which might exist between his suit and his skin or into his mask would decrease and this could cause him to suffer a skin squeeze.

This way, skin squeeze can happen as a consequence of a change in volume in air enclosed against diver's skin. Most of times, when this happens at an area covered by diving suit it is called suit squeeze and usually happens to divers who are wearing a dry diving suit and it becomes tight around a specific skin area due to air volume reduction.

The hopper/dropper combo, takes same principles but applies them to a dry fly. Using a larger dry fly with lots of buoyancy, and gooped up with Gink, you then attach sub-surface dropper pattern that will have you fishing sub-surface as well. If you are using a buoyant enough dry fly, then you may even use a small split shot above your sub-surface pattern, to help it stay down.

Rigging Dropper

There are fancy ways to attach dropper flies to your tippet, some use tag ends of tippet, to attach dropper fly, others use loops on tippet itself. But for me there are two very simple ways to rig dropper. After you have tied on your fly at end of tippet, you can easily attach about two feet of tippet through eye of that fly, and then attach another fly and other end of this tippet. This method does not work well if you are planning on attaching a split shot above second fly. The other method involves rigging up normally, and then at end of bend of fly already tied in, tie in a two foot or so section of tippet, and then tie in your fly at end of this section. The drawback of this method is, it is easier to lose your dropper fly, especially when using barbless hooks.

Cast these riggings with care, as it is easier to tangle your flies. Once tangled it is often easier to cut rigging off and redo it, rather than try to untangle two flies. Also cast very tightly, as it can become unwieldy with wild casting or under windy conditions. The last warning is in regards to releasing fish, always make sure both hooks are out of harms way, before releasing your fish. It is very easy during release to snag fish, your hands or your legs/waders, with free hook.

Dropper riggings are great for searching for fish, they take a little practice to cast and to tie. But rewards will quickly outweigh time invested to learn proper way to do it.