[ohwp]

With bi-directional path tracing it works something like this:

  * trace a photon from the light source with x bounces
  * trace a ray from the camera with x bounces

Now the end points of those traces are not connected but you can calculate the probability that they can 'see' each other. You can use this calculation for the light transport from the light source (photon) to the camera (pixel).

Edit: maybe this image will make it clear: http://lebedev.as/web_images/historyGlobal/15.png

The 'deterministic step' connects the raydiance from the light to the pixel.