Spine is straight, shoulders are relaxed, elbows are bent towards the earth. There is no tension anywhere in the body. All classes begin and end with several minutes in this position. There is no shame in riding a flat road. This is probably the most powerful, physiologically challenging, and deeply rewarding position in which to ride indoors.
Weight is on the saddle and feet, with the hands gripping and pulling on the handlebars.
Butt is slid a bit further back on the saddle in order to further cultivate the use of glute and hamstring muscles. Lower abs are tucked in so as to push the butt down and back into the saddle. Arms, back, and shoulders are utilized to pull on the handlebars to assist the lower body in keeping the pedals constantly rotating in a smooth, regular cadence. If riders are getting out of saddle to apply body weight to the pedals during a long seated climb, then they are using too much resistance and probably have a rounded back, relaxed abs, and a bogged down cadence. They should be encouraged to stay with the seated position and keep their momentum up by using somewhat less resistance.