What's the best way to start an 8 year old on Lego Technic building?
The best way would definitely be to appeal to his already existing preferences and show him how LEGO Technic can integrate into that or even enhance that.

Does he like cars? Let's build a simple car with him, later add a steering system, and still later a motor for driving.

Does he like robots? Offer to build a robot, with gears to move the arms, legs, head, etc. These can be motorized later as well.

The same can be done with planes, boats, farm machinery, animals, almost anything he might fancy.

You mention Crazy Action Contraptions, you could sit down with him and build some of the models included. Along this process you'd be able to gauge his interests both by area (i.e. does he like the Speedster or the Grabber more) and by general attitude (does he even like the whole building, or is he just waiting for you to finish it so he can play, or does he even care about it at all).

As alluded to in the last sentence, you should be mentally prepared for the possibility that you might not be successful, as he might not be interested in the topic. Don't take this personally, we all have different interests and forcing things usually don't turn out well.


  just don't understand if there is a difference between so called original bricks and Lego Technic pieces


This sentence hints at your own inexperience regarding the subject, maybe in this case you can approach the topic as a common learning process, where you both venture out to explore the world of LEGO Technic. Alternatively, you could get familiar with the terminology, basic building techniques and principles so that you can coach him. Either approach is viable, they just require different attitudes and preparations - you can switch between them however, if the need arises.