When you are Training or riding your horse, you have three main aids available to you. They are Legs, Hands (voice being a secondary aid) and Weight or Seat.
The Weight aid influences the horse at all times. It is the Central Point or Centre of gravity of the riders' seat. This includes the riders Chest, Shoulders, Pelvis and Thighs. To have an effective seat or weight aid it is important to have both good body control and muscle tone. You can control your horse with your seat if you have these 2 things. The quality of your horses' response to your other aids can greatly depend on the position and effectiveness of your seat or weight aid. You should be able to ask your horse to slow down for speed up and even turn.
When your weight is incorrectly distributed it can affect the horses' balance and prolonged incorrect weight distribution can cause poor muscle development. Your horses' paces can become uneven and your horse may develop a permanent lameness or injury.
Correcting the imbalance should start with fixing your position. If you feel that your horse is crooked and there seems to be no reason why, assess your position first. Developing healthy core strength and being aware of what your body is doing is the key to correcting any imbalances in your own body.
This takes time and sometimes a lot of physical effort from you off the horse. However if you are aiming to be a professional rider, then it is the most important skill to have. Equestrian these days, having the best horse is not how to win, or being the gutsiest rider doesn't get you the gold. It's the Rider and Horse combination that wins the World Equestrian Games, the pair with the best bond, and the best horse/rider combination.
Good Balance and Good body control is essential to give a correct weight aid. The more sensitive you can be with your weight aids the more sensitive you can teach your horse to be. On the other hand, if you are heavy or rough with any of your aids then you horse will become unresponsive and learn to ignore your aids.
You need good, strong, correct aids to effectively ride and Train your Horse. If you start with the basics, then when you start to Train your horse more difficult moves it will be easier and flow a lot better.