Phasor (complex plane): an arrow of fixed length at angle (2πft + θ). It spins counter-clockwise at the signal's frequency.
Wave (time domain): the horizontal projection of the spinning arrow, traced out over time — a cosine.
Drag the phase slider: the arrow's starting angle rotates, and the whole wave slides sideways by the same amount. A 90° rotation = a quarter-cycle shift. Same number, two pictures. This is exactly what a constellation point encodes.