The crucial step in proving the Pythagorean Theorem using the described diagram is:
The blue square on the right contains the yellow square + the green square on the left.
This statement fundamentally captures the relationship between the areas represented in the diagram, illustrating how the areas of the squares on the legs of the right triangle (yellow and green) relate to the area of the square on the hypotenuse (blue). This aligns with the core principle of the Pythagorean Theorem: \( a^2 + b^2 = c^2 \).