The effect of spatial resolution on local propagation velocity of the Turbulent/Non-Turbulent Interface (TNTI) is documented. A TNTI-local coordinate system and a Reynolds decomposition based on a TNTI-conditional averaging operation are introduced and used to assess the local velocity field and the mass and momentum balances at the TNTI layer. A clear jump of the TNTI-averaged Reynolds shear stress exists through the TNTI layer and the TNTI-averaged fluid velocity normal to the TNTI is determined by the correlation between the TNTI normal vector and the fluid velocity fluctuations around the TNTI-average fluid velocity. The TNTI-local fluid velocity field is compressive normal to the TNTI and stretching in the direction tangent to the TNTI, a structure that has important consequences.