PStrn is the shader chooser for the PSDR tool.

This tool opens a window listing all shaders in the current Maya scene. The transparency of the highlighted shader in this list is then controllable when clicking and dragging the middle mouse button left and right in the PSDR tool. (This tool is particularly useful on bone shader when rotoscoping). When the middle mouse button is released, the transparency for that shader is then set.

Be sure to only use PSDR when needed and switch back to the select tool (hotkey: Q) when done. PSDR can have unwanted effects elsewhere in your Maya scene.