What does the "SmartRendering" feature do in MainConcept Encoder 1.4?

Here is the entirety of what the manual says about it:

"The option "Use SmartRendering if possible" enables smart rendering during the encoding process, if the source file allows you to do so."

What does this mean? Under what conditions would the source
file prevent "SmartRendering" and what would it do to encode
times? I am seeing very long encode times and I am wondering
if this setting has something to do with it.

Thanks,
vcddude