In mitochondria, what is the mechanism by which electrons are transferred between different cytochromes?
All of the members of the Electron Transport System (ETS) are organized in "complexes", clusters of enzymes and other proteins. These complexes are in proximity to one another such that electrons can be passed between them. The different cytochromes, and other electron handlers, have different affinities (attractions) for electrons. Those at the beginning of the ETS have less affinity for electrons than those at the end. The reason the electrons move through the ETS is due to that greater attraction of electrons by the subsequent complexes in the pathway. These complexes are positioned with respect to one another so that no "short circuits" occur; the electrons skip no steps down the ETS. Finally, the "low energy" electron from the last complex is mopped up by that great electron-hog oxygen.

The energy lost by the electron as it passes between carriers is not directly harvested to pump protons. The importance of this "loss of energy" is that it makes the transfer of the electrons a spontaneous process insuring their passage down the ETS. Typical entry into the ETS involves NADH donating a hydrogen ion to the matrix and a pair of electrons to NADH dehydrogenase. The electrons are eventually transferred to the mobile coenzyme Q. This now attracts a pair of hydrogen ions from the mitochondrial matrix which then induces the operation of a proton pump. This pumps hydrogen ions into the inner-membrane space creating the electrochemical gradient. 

So, in general the arrival of electrons, with the attendant hydrogen ions, changes the conformation (shape) of pumps which translocate the protons to build up the gradient. The electrons are ultimately passed on to oxygen, forming water with the help of some spare hydrogen ions. This removal of the electrons allows the ETS members to be receptive to another load of electrons from the NADH.

Reference 2nd paragraph: http://www.physiologymodels.info/metabolism/ETSOXPHOS/ets.htm
This is a good site to read more and view some diagrams, especially focus on the second panel, "Complex I".

Reference 1st paragraph: Molecular Biology of the Cell; Alperts, Watson, et.al.