SketchUp has a feature where you can tap any of the arrow keys on your keyboard to lock inference. For example, grab the Line tool, click once to start the line, then tap LEFT arrow key to lock GREEN axis, RIGHT arrow key to lock RED axis, or UP arrow key to lock blue axis.
This is much faster than having to find the reference manually by aligning the mouse, then holding SHIFT to keep the inference. It would be great if PlusSpec could keep this native feature of SketchUp when using the various custom tools.
(tip number 4. )