-zones=<options>.

Allows to override the quality / bitrate for a certain part (ending, credits, ...). 
The options are <start-frame>,<mode>,<value>[/<start-frame>,<mode>,<value>[/...]]

mode = character (q : constant quantizer, w : ratecontrol weigth)

value = float, quantizer vale (2.0 - 31.0) for constant quantizer mode or 
quality correction in % (0.01 to 2.00) for weigtht mode

Example: encode all frames starting with frame 90000 at constant quantizer 20:

zones=90000,q,20

Example 2: encode frames 0 - 10000 at 10% bitrate, encode frames 90'000 and 
up to the end at constant quantizer 20

zones=0,w,0.1/10001,w,1.0/90000,q,20

(that second zone is needed to delimit the first zone.. without it 
everything up until frame 89999 would be encoded at 10% bitrate)