The Same-Groove Same-Speed Law states that the KuiQuark flow velocity at a fixed Galactocentric radius R is single-valued. Passive stars (diskless, slow rotation) strictly follow this flow, producing minimal tangential velocity dispersion. However, observed dispersions are not zero. We propose the Thickness Surfing Law: the vertical thickness h(R) of the Galactic disk provides a degree of freedom. Stars can move to different heights |Z|, converting tangential velocity differences into vertical motion, thereby avoiding collisions. Using Gaia DR3 data, we select passive stars at R = 8 ± 0.5 kpc and compare two populations: low-Z (|Z| < 100 pc, N=412) and high-Z (200 < |Z| < 500 pc, N=297). The tangential velocity dispersions are σ_φ = 6.31 ± 0.32 km/s and 12.87 ± 0.55 km/s respectively. Median ages are 2.1±0.3 Gyr (low-Z) and 2.3±0.4 Gyr (high-Z), consistent within uncertainties, ruling out age-driven heating. Binary contamination is small (<5%) and independent of Z, not biasing the comparison. The larger dispersion at high Z directly demonstrates the Thickness Surfing Law: the vertical dimension absorbs tangential velocity differences. Without thickness, stars on the same groove would collide and merge, erasing all dispersion. The observed dispersion is not noise but a measure of vertical freedom. This paper is part of the XLG-CM series (XLG Cosmology — Celestial Motion). Part of XLG Cosmology — Eccentric Collision Universe (ECC).