A good ski exercise program will work both strength and balance
In fact, it doesn’t help you much to have real strong legs if your balance is left untrained. You can do all the leg presses you want but have a poor balance, that will be your limiting factor and vice versa.
That’s why I believe in combining strength and balance in the same exercises when training for skiing, so that you get used to using your muscle strength in unstable situations.
While training in the machines in a fitness center has its purpose for building absolute strength and muscle mass, you should try to incorporate exercises where you involve that element of balance as well. It teaches the muscles to work together as a “team” rather than as “individualists” and the result is much better ski balance.
Good balance is when your body has learnt to recruit its muscles to act or react in a certain order. If that doesn’t happen you will not be able to recruit all your force. The system relies on feedback from little receptors out in the body that will signal that you are in an unstable position. On order for this system to work well the communication channels have to be used regularly.