Jeff Cavalier explains that to maximize the effectiveness of a back workout, one should focus on effective reps rather than just completing a set number of repetitions. He introduces the concept of an 'ignition set,' which is a set performed to near failure to activate the muscles. After a short rest, the focus shifts to performing effective reps, which are the reps that truly stimulate muscle growth. Cavalier demonstrates this with exercises like the seated cable row and lat pulldown, adjusting the weight and rest periods to maximize muscle engagement. He suggests using a 12-rep max for some exercises and an 8-rep max for others, depending on the desired intensity and muscle focus. The goal is to accumulate a specific number of effective reps, such as 20 for a 12-rep max or 10 for an 8-rep max, across multiple sets with minimal rest in between. This method is designed to enhance muscle hypertrophy by focusing on the most challenging and beneficial reps of each set.