http://www.stackyard.com/news/2007/07/machinery/06_case_steiger.html

More Power, Fuel Economy for Case IH Steiger Tractors
30/07/07

"Turbo Compound involves two turbo sets working in a series," Dean explains.
"The first turbo performs as a conventional turbocharger using exhaust gas
to compress air into the combustion chamber. In the Turbo Compound system,
instead of routing the exhaust out the exhaust pipe, it then goes to a
second turbine downstream from the turbocharger. The energy from this
second power turbine then transfers power to the crankshaft through a
set of reduction gears and a hydro coupling. This provides fuel efficiency
gains of up to 3 percent. The result is increased performance, horsepower
and fuel economy."

"Dean adds that while Turbo Compound technology has been used in some
on-highway applications, "We believe this is the first time it's
been used in an agricultural tractor and will mean outstanding
fuel savings for the customer."

What is achieved here is far below what Detroit Diesel achieved with
the DD15 turbo compound diesel truck engine.

It is always hard to improve a diesel as they are better to begin with.
Of course, unlike a rotary, it still has exhaust valves to retard the
flow. In addition there is additional heat loss by using two turbines.
The first turbine and plumbing loses heat to the second  turbine above and
beyond what it needs to turbo charge the engine.

In theory a single turbo charger, with drive shaft, would be more efficient.