wrote:

> Well, it's not exactly the same, because you can, by increasing the
> pressure, increase the speed of the wheels, but how can you, by
> reducing the pressure, slow it down?

I think you misunderstand how a torque convertor works. The engine
drives an impeller that drives fluid through a turbine that is attached
to the input shaft of the transmission. The moving fluid (not "pressure"
per se) drives the transmission. Under engine braking, the roles reverse
and the driven turbine now acts as the impeller.

It is less efficient working "backward" this way, but it works and
provides engine braking just fine.