It is a technology using two spark plugs in a engine (very simplistically speaking) you may find some very good information in  and .
There is a lot of information on this on the web,  and  will get you started
I would avoid going deep in the theory part and focus on what the patents say.
Patents are a very important tool, they can give you the insights in what went through the inventors mind when they worked on the technology and what is the invention level (as in TRIZ level) etc
In the related articles which will be updated soon:
- We will start by analysing the basic patent or the original patent for the DTSi technology. WO2005042954  titled 'IMPROVED INTERNAL COMBUSTION ENGINE WORKING ON FOUR STROKE PRINCIPLE' (The basic patent of DTSi in reality is an Indian patent from which this patent takes priority, I havent found that patent yet so this one is the next best choice)
- In our analysis we will identify the claim elements and present them here in simple language
- We will try to identify its invention level as per TRIZ