DTSI is abbreviation of Digital Twin Spark Ignition that shows there is dual sparking technology. Which means there will be two spark plugs used to start combustion process of fuel. Bajaj Pulsar is example of DTSI bikes.
Where as DTSI-SI stands for Dual twin spark swirl induction. In this type also there will be two spark plugs used for combustion of fuel. The only difference between these two technologies is, in second one swirling of the combustion takes place which is not the case in first one. Bajaj Discover is example of DTS-SI of bikes.