Silhouette Effect

Hey there, fellow icon makers! I've been trying my hand at trying to incorporate this silhouette effect (as demonstrated in the icon below) into my icons but I just can't seem to get it right on my own. I remember reading a tutorial about this somewhere but can't for the life of me recall where...


Icon by sallyna_smile @ castlesandfire

I know the top image is set on screen/lighten but can't figure out how to get a clean, crisp silhouette from the bottom image... Any ideas? Any help is greatly appreciated!