Cross over mirroring is when you match a part of your client's physiology with a different part of your own. For example: tapping your finger in time with a client's breathing rate.
It is used as a more subtle form of mirroring — particularly useful in situations where direct mirroring would be too obvious or inappropriate, such as in a coaching conversation or a formal setting. By matching the rhythm or pattern of one behaviour with a different physical channel, you can still build deep rapport without the other person becoming consciously aware of what you are doing.
See also: List 5 things to match to get rapport | What is NLP?