An overlay of the source is displayed within the brush.
Working this way is actually quite difficult because the source significantly
blocks your view of the destination. But if you prefer it, try reducing the
opacity of the overlay so that you can see the image below.
Vanishing Point
Vanishing Point takes cloning to an entirely new dimension, literally. The
tool makes it possible to set up primitive planes across your artwork, which
a clone then follows to simulate a three-dimensional space. Vanishing Point