Two-Finger Sniff:
This picture shows what I've been calling the "two-finger sniff". I like to do this as a way of introducing myself to cats to see if they want attention (or not). Many people offer 1 finger to a cat to sniff; I prefer 2 because it seems to be less confusing to cats. 1 finger sometimes can be tempting for cats to play with; all 4 fingers can be intimidating to cats that are fearful; 2 fingers seems to work the best for me.
In this picture, you can see my cat Yato eagerly accepting the two-finger sniff. Even with cats you know well, offering them finger-sniffs before petting can make them more relaxed and comfortable with you.
For a video demonstrating the "two-finger sniff", please click here.