When the present style fits the group's readiness, performance tells the leader when and how far to change: rising performance means task-relevant readiness is growing and the style should shift along the curve toward less direction (and later less support); declining results are the cue to shift back toward more direction and support. The SLII partnering steps make the same point by identifying the performance trend for each goal when the development level is agreed.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.