Xíngshū — running script
What running script adds — filaments, fused strokes, axis — and what it is not allowed to take away.
之 in running script: what the 牵丝 is allowed to carry
之 is three strokes in regular script and looks like one gesture in running script — but the change is not simply speed. Running script adds a filament (牵丝) between strokes that must be thinner than any real stroke, and 之 is the shortest possible place to learn where that filament belongs and where it does not.