In low Reynolds number regimes, sperm cannot rely on simple reciprocal motion for net propulsion, yet the arXiv preprint 2605.29050 demonstrates that at the boundary between a viscoelastic fluid and solid substrate, the flagellar wave direction and cell trajectory decouple. Using high-resolution video microscopy, the authors tracked individual bull or human sperm (exact sample size unreported) and found that head orientation oscillates in phase with movement direction while the tail wave lags, producing larger amplitude turns than wave propagation alone would allow. This dynamic weighting—reconstructed as a time-varying linear combination—suggests the head acts as an active rudder modulated by substrate contact forces. The final flagellar bend remains nearly stationary in the lab frame, imposing a semi-holonomic constraint linking sliding velocity, wave speed, and forward progress. Classic resistive-force theory (Gray & Hancock 1955) assumed fixed head-tail coupling; recent work on cervical mucus rheology (Suarez et al., Biol Reprod 2018) shows sperm must navigate shear-thinning, elastic media where such constraints become decisive. The preprint therefore reframes motility not as pure tail-driven thrust but as an emergent negotiation between head-substrate friction and flagellar bending, a nuance missed by purely hydrodynamic models. Limitations include the artificial glass-fluid interface (no in vivo oviduct geometry) and lack of peer review; the 2026 preprint status means claims await validation. Fertility implications are direct: disrupted head-tail phasing could explain certain idiopathic asthenozoospermia cases where standard CASA metrics overlook directional instability.