The Weight hammer tool lets you quickly fix vertices with weights that are causing spiky or otherwise undesirable deformation. Clicking the Weight hammer assigns selected vertices with weight values from neighboring vertices.
To fix vertices with undesirable weights
Paint Weights Tool: A. Modes B. Weight Hammer
Use the hotkeys Ctrl + < and Ctrl + > to quickly toggle between these modes.
Hammer Skin Weights assigns the selected vertices the same weight values as their neighboring vertices. When you exercise the skeleton, you see smoother deformation results.