<p>The physical mechanisms that drive changes in the velocity field and skin friction caused by a thin plate immersed in a turbulent boundary layer (TBL) are numerically examined via LES. The TBL develops in a water channel at Reynolds number Re_<i>θ</i> = 540–600 based on centreline velocity and momentum thickness. The plate’s chord and width are equal to <i>c</i> = 0.53δ and <i>l</i> ~ 1.3δ, respectively (δ is the thickness of an unperturbed TBL at the plate location). A wake (a velocity deficit region bounded by shear layers) and a pair of edge vortexes are generated by this plate. Their effects are studied at four plate locations in the inner TBL area. The effect of the wake is considered separately for the velocity deficit region and its shear layers. The velocity deficit region isolates the wall flow from the rest of the TBL. Thus, the velocity gradient at the wall is determined by the flow velocity below this region rather than at the center of the channel. The expansion of this region downstream causes a further decrease in the velocity gradient. It becomes minimal, when the lower shear layer of that region disappears. The shear layers of the wake and edge vortexes create normal and spanwise flows that reduce the wall flow velocity and thereby skin friction, but their effect is secondary. Edge vortexes reduce the surface friction beyond the plate width and have almost no effect on the friction in the channel center until the lower shear layer degenerates. The total drag introduced by the plate into the flow exceeds the reduction in skin friction.</p>