
The plane you want to sketch on is sometimes obscured by geometry or hidden inside a part model. When the Sketch tool is active, use the Slice Graphics option on the context menu to slice away the portion of the model temporarily that obscures the plane on which you want to sketch.
You can use an existing plane or, in an assembly, create a work plane as the cutting plane .
Rotate the model so that the plane you want to sketch on faces toward you. You select the plane you want to sketch on, then slice the model. The portion of the model facing toward you is sliced away, and you can create sketch geometry as needed.

In an active sketch, you can temporarily slice away the portion of the model facing you to reveal the sketch plane.