Tongue flicks are widely assumed to be appeasement behaviors, and are designed to pacify or avoid aggressive behavior from the other dog (or person). This is different than submissive behavior, which is designed to defuse aggressive behavior that is already in progress from the other dog or human. Appeasement can also indicate a desire for interaction, or, as espencer suggests, a desire to increase distance, but it's not always done out of anxiety, sometimes it's also done as part of greeting to acknowledge the status of the other. So, in that sense, yes, the dog may feel a violation of personal space, but not be "anxious" in the context of fear. However, sniffing the ground, shaking, and yawning are displacment signals - those are signals that are out of context and that the dog uses when stressed or frustrated, and are not signals to others, but, rather a way of distracting themselves or doing something when they don't know what else to do in a frustrating situation. Lots of times, it happens during play, when dogs don't really know what to do next when the play escalates beyond what they are comfortable with.
Cut-off displays are those which signal to the other dog that no further interaction is desired. Tongue flicks can be used here, as well as curving the body away, turning the head away, turning the back, averting the eyes. Teaching the dog to curve its body on cue can be very useful, as can teaching it to do a turn away with the head. Curving the body says "I'm not a threat".
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"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George
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