This phenomenon is a highly sensitive, non-specific gauge of interest between two people who are communicating for the first time via email. It is defined as the gradual lengthening of email responses with each back and forth reply. When present, this phenomenon usually indicates some level of mutual interest between the communicating parties; while the respondents become more and more engaged, the responses subsequently lengthen. The absence of this phenomenon does not imply that there is no level of interest.
Disclaimer: This phenomenon does not predict actual compatibility, only initial interest.
Presence of the Ping Pong Phenomenon (or a positive(+)PPP):