Hold down 
+ 
(Windows) or 
+ 
(Mac), click the curve and
drag along the curve.
When you are sketching, you can snap a brush to a curve (or surface isoparametric curve or curve-on-surface). This allows you to use curves as sketching guidelines.
button to the right of the
prompt line, or hold down + (Windows) or + (Mac), to activate curve
snapping.
The brush profile will jump onto the curve.
❒, then choose Paint from
the left side of the option window, and set Brush
Curve Snap Tolerance to the number of pixels the brush
cursor must be from a curve in order for the brush profile to jump
onto the curve.
button again, or release + (Windows) or + (Mac), to de-activate curve
snapping.
) or curves on invisible
layers (Layers > Visibility > Invisible 
).
