A hard edge stops the viewer’s eye and draws attention to a particular area in the painting, while a soft, ambiguous edge allows the eye to travel, unimpeded, from one shape to the next. Some edges are varied, alternating between ambiguity and clarity many times along their length and causing the viewer to change their focus and rate of looking accordingly.
For a crisp edge, load your palette knife with paint, then lay it flat onto the canvas and pause before continuing the stroke away from the edge you’re trying to create. That momentary stillness will deposit a greater volume of paint than a swift, continuous stroke, emphasizing the edge.
A soft edge is made by slurring the paint together at the intersection of two colors. Press the palette knife onto the boundary and move it back and forth to work the patches of wet paint into each other. The colors will merge, de-emphasizing the edge and allowing the eye to travel across it more easily.