The principle of is shown on the following animation.
The concept shown works as a 2 cycle engine. The 2 cycle regime is vital in this case as there is a need to release the combustion energy during each cycle, otherwise the mechanism fails.
The 2-cycle engine is often associated with notorious eastern European block cars Trabant, or motorcycles Jawa Pioneer with all its limitations. In our case however we are talking about a lot more advance technology using an Air Assisted Direct Injection System (technology Ditech). This eliminates most of the cycle limitations. On top of that, in some circumstances, the 2-cycle regime has a higher thermodynamic efficiency and output ratio than a more wide spread 4-cycle. Even though, not officially known, the high powered marine engines are almost exclusively designed as 2-cycles.
The concept shown in our animation is not the only possible free piston set up. Two pistons on each side can also be used allowing for a 4-cycle design.
Also multiple engine design would help a better balance and elimination of vibration. If we use two engines in a line and set them in opposite directions, then we will get a theoretically perfectly balanced set up.