Again, the energy loss due to compression is negligible because most of it is used on pushing the piston back down (only some is lost in form of heat).
The only thing that actually matters is the amount of energy it takes to suck (or to try) vast amounts of air through the nearly closed throttle (or to push it through the exhaust valve on such Diesel engine). So the more the throttle is open = the longer can the vehicle coast (and conserve fuel). And the weaker such braking becomes.
Diesel engines have that valve built in because without it, there would be no real engine braking to talk about.

So what is worse,

creating a vacuum when throttle is closed and piston goes down and no compression when piston goes up?????

or

sucking air in when throttle is open and piston goes down and creating compression when
piston goes up......

b.t.w. theres no up and down movement at a 2cv only in and out....

It's been ages since I have had and in and out movement ....

laxative chocolate or senna pods 2 chevs

sorry this is bad I know; but a 2cv is double Dutch then........

Any in and out movement came to end after 1-2 children...

What I'm trying to say is that it depends on what You're trying to achieve. If it's economy, longer coasting will require a larger throttle opening, because it offers less energy loss. If it's engine "braking power", some kind of a closed inlet/outlet valve (be it in the intake or exhaust part) will provide the mechanical resistance . Creating that vacuum is what slows You down, NOT compression of air.
Because any real braking is done by, well, the actual braking system, I think the right approach is to go for the economy. Although not to the extreme of, say, opening the throttle fully, because engine braking, as subtle as it is, is still useful in counteracting a limited amount of understeer and providing means of steady gradual slowing down.

Saurus,
Fiat seem to have got to grips with this subject...

"With no conventional throttle butterflies in the inlet manifolds, the engine is relieved of the pumping losses which dog conventional petrol units."

From; http://www.telegraph.co.uk/motoring/gre ... works.html

Back to 2cv.
The only thing which works for a Visa carb is that the butterfly must be closed and the electric idle fuel valve must be closed. With an open throttle the mainjets are jetting fuel
so there is still fuelconsumption.......

Maybee a separate iddlevalve as mentioned before can help.... letting air in when throttle is closed???

This is brilliant. Complex, but brilliant. Limiting the amount of air on intake by duration of valve lift instead of a choke, that might actually be something new (or as usual, it was invented some 100 years ago but without any means to make it work;) ).
I still think any real and meaningfull progress in "people's cars" ended in the 90's, when cars were advanced enough to be useful, efficient etc. and not complicated enough to become a pain in the ass for the owners

@lpgo: I guess You're right about that if we're talking about carbs, and a separate valve past the carburettor wouldn't mess with the air flow in the carb itself.