Adobe Premiere Pro CC v.7.xx - Manual (Page 513)

Twitter™ and Facebook posts are not covered under the terms of Creative Commons.
Legal Notices | Online Privacy Policy
Note:
Exponential Fade fades out the first clip over a smooth logarithmic curve while fading up the second clip, also over a smooth logarithmic curve.
Selecting an option from the Alignment control menu, you can specify the positioning of the transition.
Though the Exponential Fade transition is similar to the Constant Power transition, it is more gradual.
Adobe also recommends
516