A feature in the V.92 modem protocol to allow more 'balanced' data connections. Under a regular V.90 connection, the downastream connection is 53-56kbps, while the upstream connection is still limited to the V.34 of 33.6kbps.
Utilizing the V.PCM system, the transfer speeds become symetric at 48kbps each way. Along with simply giving a bigger upstream pipe, this has another unexpected side effect, as any transfer that requires active acknowledgment (TCP-based streaming media as an example) will potentially run much faster due to the larger bandwidth the ack messages can be sent on.