What does ski-doo's SDI mean?

Ski-Doo SDI - a quite different and more economical route to low two-stroke emissions and fuel consumption. SDI stands for Semi-Direct Injection, and it describes a system by which fuel is injected, not into the combustion chamber, but into the fast-moving air streams in the rear transfer ports. Because injection into the transfer streams more than doubles the amount of time available for evaporation of the fuel droplets, specialized fuel injectors like those of Orbital and Ficht are not needed. Direct Injection must wait for the exhaust port to close before injection can begin, but with SDI, keeping fuel from reaching the exhaust is accomplished by the natural scavenging flow pattern of a high-powered two-stroke.