If done properly, this gait can be very fun and comfortable to ride. You need to have a strong core and open hips to stay in the saddle. The more you bounce in the saddle, the more the horse’s back become hollowed and difficult to sit. We want to think about keeping our body tall, knees open, and sink down into the saddle. By starting to learn to sit the trot on a circle by frequently changing your diagonal you keep the horse’s back supple. You can increase the frequency of sitting as the rider becomes more comfortable and synchronized with the horse. This video gives a lovely demonstration of the sitting trot.

Source: https://www.youtube.com/watch?v=qRVf-NvZUgY