Quantizer compression
If the value is set to 100%, the quantizer will stay nearly constant (high motion scenes will look bad). If it's 0%, the quantizer will be changed to make all frames approximately equally sized (low motion scenes will look bad).

Quantizer blur
Blurs the quantizer graph over time: 0% - no blur, 100% -  average all past quantizers.

Max quantizer difference
Limits the maximum quantizer difference between frames.

Minimum/Maximum bitrate
Unlimited if set to zero.

Decoder bitstream buffer size
For MPEG1/2 this also sets the vbv buffer size.

Descriptions by Michael Niedermayer