This diagram describes the behavior of the device when excited by repeated pulses of given length and duty cycle. The x axis shows the pulse length [sec], the duty cycle is the curve parameter. The y axis shows the average temperature elevation in terms of an "effective thermal resistance" [K/W]. In case of a stream of very short pulses the power source can be interpreted as an effective source with a power downscaled by the duty cycle. In case of very long pulses we get the original heating curve again. This curve is extremely useful for producing data sheets for devices and designing switching type power supplies.