Dorian trains for only 45 minutes, but it’s Intense! You can feel it ! He uses three exercises per bodypart, three sets each. The first two sets are short of failure, while the last one is an all out effort to failure with assistance from his training partner.
High Intensity Training (HIT) is a form of strength training popularized in the 1970s (first by Arthur Jones of Nautilus and Medx fame).
Its principles are:

-progression (the trainer adds more weight to the bar/machine session each session as they attain greater strength)
-overload (where the muscles are forced to hypertrophy or thicken by being exposed to above-normal levels of stress)

More weight/repetitions are added to the bar/machine in consecutive workouts as the trainer adapts. It may be summed up with the philosophy of ‘intense, brief, and infrequent’.