This sub-threshold intensity steady state session uses the peripheral heart action protocol where upper and lower body exercises are alternated for extended periods of time at low intensities. Alternating the exercises allows for recovery to the opposing body segement, while prioritizing blood flow to the working area. Even though strength exercises/movements are used, the objective is cardiovascular in nature. Try to sustain the activity for a total of 30min with the intensity around a 5-7 of a perceived exertion or 60-75% Max HR.
It is important to stay below the lactate threshold (or the point a which exercise becomes hard) as a specific training stimulus is prescribed with the ensuing adaptation in order to maximize our bodies ability to metabolically flexible. Moral of the story is just because you "can" train harder it is not the purpose of this prescribed session.
Benefits of SISS are improved overall cardiovascular/cardiopulmonary function, increased mitochondiral density (cellular "powerhouses") and improved aerobic metabolism and fat "burning" capabilities.