Overlap is the process of leading someone from their preferred representation system to a less preferred one.
It is used when a person has difficulty accessing a particular modality — for example, someone who says "I can't do pictures" may have an underdeveloped Visual system. By using overlap, the practitioner guides them from a modality they are comfortable with (e.g. Kinaesthetic) into the Visual system.
Example: "Imagine you are standing on a beach. Feel the sand against your feet. Hear the water lapping against the shore. Listen to the birds overhead — and now look up, and notice if you can now see the birds."
The practitioner moves from Kinaesthetic → Auditory → Visual, gently leading the client into the less preferred system.
See also: What are the Representation Systems? | What is NLP?