The person on top straddles the legs of the person on bottom. Visually, at least in my mind, it looks as if the person on top is doing a wide-legged push up. Expect to sweat. Neither person is putting weight onto the other person. As the person thrusting is perpendicular, his or her weight is used more efficiently.
Sex is exercise and does burn calories.
While this position requires a bit more co-ordination than the missionary position, it is often considered less intimate. However, this position does allow for great foreplay. Again, the difference between this position and the missionary position is that this time, the partners face the opposite direction of each other. Each person has a view of the other person's feet. The angle of entry is different and often will require verbal communication.