Rocking action at the foundation-structure interface has long been proposed to isolate structures from strong ground motion. In this paper, the fundamental concept of rocking isolation is examined in depth to guide further design efforts. This is achieved by first deriving an analytical model of a flexible structure freely rocking on rigid ground. Decomposing the coupled equations of motion of the model into their modal components provides new information on the mechanics of rocking isolation. After identifying the salient parameters needed to quantify rocking isolation, equations to predict the lateral accelerations, base shear and overturning moments arising during rocking are provided. The analytical model and the simplified equations are then validated using some of the earliest experiments on rocking structures, which were completed in New Zealand. These validations clarify poorly understood phenomena concerning rocking isolation, such as how rocking and vibrations of the structure couple, how this influences the excitation mechanisms of the structure, resulting in seismic shear forces and overturning moments larger than those required for uplift. The findings provide an analytical basis for designing efficient rocking systems that successfully limit force demands.