How is the Leslie speaker simulated?
I highly recommend taking a closer look on great book:


  Zolzer U. - DAFX Digital Audio Effects


As well as his publication on this specific topic:


  Disch S., Zolzer U. - Modulation and Delay Line Based Digital Audio
  Effects


Leslie (Rotary) speaker can be simulated using tremolo and Doppler effect for both change in signal amplitude and pitch. 

Basic signal flow diagram that can be found in the article: