The 'Detect Edges' feature of Aperture's brushes is very powerful. Edge detection is based on tonal differences in the photo. By default, the brush continually takes tonal samples as it's being applied. The "+" or "-" in the center of the brush is the current sample point. However, in photos where the tonal ranges across edges are close, Aperture can have trouble detecting edges.

In those situations, you can help Aperture's Detect Edges by using the Shift key. Hold down Shift and the brush uses the first click as the only tonal sample until the next click. The video below shows an example of this in action.