If you want to know more about it than you ever wanted to know I can recommend the books by Audsley. It contains a very detailed description of both systems.
Tubular pneumatics works about as follows:
- you have a small valve under each key
- the output of this valve is connected with a tube to a larger bellow under/inside*the windchest
- if you push on a key, then the bellow under the*windchest*opens*the valve under the pipe and the pipe speaks.
(this is over-simplified but you get the gist)
There have been systems with pressure and vacuum etc. The advantage at that time was that you could bridge large distances and still have a light action since you only had to actuate that small valve under the key. Also all imaginable couplings and pre-sets were possible, something could not be done with mechanics (or not as easy). Big disadvantage was the huge delay between striking a key and he pipe speaking. I have a few times played on such an organ and it is horrible. You are playing with 10's of milliseconds delay.
Electro pneumatic was the next step. In that case your key closes an electric circuit that opens a small valve below windchest. That valve actuates a bellow and that bellow opens the valve to the pipe. So it is only the last step that is pneumatic. Coupling and pre-sets are electric as is the keyboard. But the last command of the pipe valve is pneumatic. There is a lot less delay as all those pneumatic tubes to the windchest are removed.
A pure electric action uses an electric valve coupled to the pipe.