It seems to be something about how when client certificates are forced to be used, the calculation of the TLS master secret can be re-calculated by the MITMing

It seems to be something about how when client certificates are forced to be used, the calculation of the TLS master secret can be re-calculated by the MITMing party (who also has the private key required for part of the calculation), and can therefore decrypt the main traffic flow between the client and the server, and also because the master secret calculation works out, the client ends up believing the server has proven its identity? Also something about "fixed DH" handshakes.