This feature available only for advanced_hls, fmp4_hls, and advanced_fmp4 output formats.
iOS 5 now supports Fast Forward and Reverse Playback. However, you don’t need to produce special purpose content to support Fast Forward and Reverse Playback. All you need to do is specify where the I-Frames are. I-Frames, or Intra frames, are encoded video frames whose encoding does not depend on any other frame. To specify where the I-Frames are, iOS 5 introduces a new I-Frame only playlist. Read more at Apple dev technote.
The Encoding.com API exposes this format of output by having the user request set the add_iframe_stream parameter to yes in a job request's stream section.