Dynamic Instabilityby David Chamberlin
I'll begin by defining what is meant by dynamic instability in the context of diving applications. To understand dynamic instability, it is useful to start with static instability. The best way to see static instability is to simply stop moving and hover without moving either your hands or feet. If you end up rotating in a particular direction (head-down, left-side down, etc.) that is static instability. It means that weight and buoyancy are not evenly distributed across your body and gear. Technically "instability" is probably not the best description of this, since once you reach the attitude in the water that is dictated by your weight/buoyancy distribution you should be fairly stable.
Dynamic instability refers to the condition where a change in position/attitude is "exaggerated". An example that is familiar to most divers is when you rotate your body left or right. When you do this, the weight of the tank is no longer positioned over your center of gravity and thus starts to contribute to your rotation, and if left unchecked will leave you on your back looking at the surface.
This same phenomenon occurs as a result of air shifting either in your drysuit or your BC. Again, a familiar example for dry suit divers is when you go head-down, the gas rushes to your feet and ends up standing you on your head. The degree to which this occurs is related to how far the gas can travel, and how much gas is present.
These principles can be most easily explained using the example of a simple teeter-totter. If you have equal weights an equal distance from the fulcrum (e.g., the bar that the teeter-totter is sitting on) then it will be balanced. If you shorten the length of one of the sides, it is no longer balanced and will be "weighted" towards the opposite side. The more the length is changed, the greater the imbalance created. To be more precise, if you shorten one side by 1/2, then you would need to double the weight on that side to keep everything in balance.
Applying this to the diving example, think of the fulcrum being the center of gravity on your body. For each person and gear configuration, this position will be slightly different. For the sake of this example, let's say it is at your bellybutton. If you shift slightly so that some gas travels starts traveling to the highest position. If you were able to limit the distance it could travel, say it couldn't go past your butt, then it wouldn't have too much of an effect since it is very close to the fulcrum. In the teeter-totter example, it's like placing a weight very close to the center of the teeter-totter. Now if you let that gas travel further, say all the way to your feet, now it has a greater "arm" and will have a greater effect, just like moving the weight out towards the end of the teeter-totter. Of course once it has that greater arm and it starts to move you, more gas will move to that position and the problem escalates until you're feet-up. So you can see, the more confined we can keep the gas, the more stable we will be. Furthermore, the more gas that is present, the more of an effect it can have, just like using larger weights on the teeter-totter.
So what practical impact does that have on diving? Well, the first is that you want to minimize the amount of gas that is in your BC and in your drysuit. This entails carrying only the minimum amount of weight required for your gear configuration. More accurately, this means having a "balanced rig". It also means you do not want to use your drysuit as buoyancy (as so often recommended by instructors and even drysuit manufacturers!). If you use the drysuit for your buoyancy, you are giving the gas a much larger distance to travel. It can travel all the way from your neck to your feet, causing much greater dynamic instability issues. Whereas if you keep just enough gas in your drysuit to maintain the required loft of your undergarments and use your BC for buoyancy, then the gas can only travel the length of your BC which is a far smaller distance. And finally it means using the minimum sized BC wing for your configuration so that the gas in the wing has the least amount of distance to travel.