Which short box series exercise should a student demonstrate proficiency in before advancing?

To demonstrate proficiency in the short box series, a student should focus on the short box twist. This exercise is crucial because it emphasizes both core stability and rotational control, which are essential elements in Pilates practice. Mastery of the short box twist indicates that the student has developed adequate strength and coordination of the core muscles, as well as the ability to maintain proper alignment during movement.

Beyond the short box twist's focus on rotation, proficiency in this exercise also involves an understanding of how to engage the body effectively while seated on the box, which can prepare the student for more advanced movements. This exercise can also help in developing the awareness required for transitioning between different exercises within the short box series.

While the other exercises mentioned—back extension, twist and reach, and leg pull—are important, they do not carry the same emphasis on the unique skill set required for rotation and stability that the short box twist provides. Therefore, it's vital for the student to master the twist before progressing to more challenging elements of the series.