Create a Magnified Cutaway Detail View in Photoshop

Zoom in to 200% or more. Create a new empty layer and move it between "Group 1" and "detail large." Activate the line tool from the toolbox (under the shape tool). In the options bar, set the line width to the same size you used for the stroke effect on the detail layers. Make sure arrowheads are not enabled, style is set to none, and color is black.

Drag out two lines connecting the two circles as shown. You may need to switch to the move tool to adjust the line placement so they connect seamlessly. Hold down the control key as you adjust the line position for more precision.