I often ask people if they are interested in taking yoga classes, in a shameless effort to self promote. For instance, I mentioned that I teach yoga to a person in the building and she replied, "Do you teach hot yoga?" I replied "No, just traditional yoga classes". Later I thought well, that isn't entirely true. I teach an Ashtanga Primary Series class and that practice is designed to build heat internally & naturally through a combination of body movement and breath. Even Hatha or traditional yoga can warm the body if the participant is engaged in the practice properly.
Hot or heated yoga takes the "work" out of burning tapas or austerities. My Kung Fu teacher and Ashtangi once explained to me that "hot yoga is like cooking your organs", that was all I needed to hear to realize that hot yoga was not going to be my future practice of choice. One appeal of practicing yoga in an artificially heated space is that the human body feels the effect of the heated environment immediately. Without taking a pose the body can start to sweat, then during practice the effect is compounded by applying movement. The body gains flexibility, and superheats internally. In yoga philosophy this may fall into the category of Tantric practices. Achieving a goal quickly through artificial means.
Another example of the body's ability to respond to external stimulus versus engaging the mind might be explained by using two subjects. Person A stands in an upright position with no external weight and person B is given two 25 lb dumbells to hold onto. Person A will not have to apply any additional work, while person B is being forced to respond to the external stimuli with muscle contraction if they want to hold onto the additional weights. Person A's body is responding to nature in a balanced honest way, while person B's body must respond to an introduced external stimulus.
The effort used during yoga practice is an individual experience. Sometimes engaging more in the practice to a degree of "feeling it" is necessary in order to realize the potential of the practice. It is very easy to just relax in a pose and think, "Is this all there is?" Perhaps exchanging "Steady, Comfortable Posture" for relaxed, comfortable posture.
In yoga practice, especially personal practice, the amount of effort put in equals the effect or result. The "Every action has a greater or equal reaction" quote, or something along those lines. If the work (energy/heat) is generated through external means, it may not have long term stability but instead offers quick results that may not be suitable for long term exposure.