The dynadisc is a simple, small piece of exercise equipment that is used in a yoga, pilates environment and also in a rehabilitation setting, which is more where my experience will come from.
The dynadisc balance disc is a small plastic disc about the size of your foot and approximately 6-7 inches in thickness. It is filled with air, and provides an unstable surface to attempt to balance on while performing exercises in a single leg position. Most often, this is a very successful and useful tool for neuromuscular rehabilitation, which focuses on improving balance and awareness.
The disc is limited in its rehab potential based on the user. Meaning, if you do not have an extensive knowledge of balance, core exercises to do than you will not really know what to do with it. But if you have a larger repertoire of exercises and perform them already, this will make the difficult single leg exercise even more challenging.
I recommend this to the person who would like to advance their exercise with more dynamic, challenging balance exercises.