First, it is important that you don't feel the ground when you sit on your cushion. If you can feel the ground, inflate the cushion using a pump with a needle (sold separately - reference: 8556016). The air pressure inside the cushion is what determines the instability level. The more your disc is inflated, the less you'll work your balance; the less it is inflated, the more you'll work your balance.

Balance is very important for all your sports activities. It's quite obvious when it comes to sports like skiing, trail running or kitesurfing: for these sports, good balance can keep you from falling or getting injured (especially by strengthening your ankles). So, balance is pretty useful, don't you think?