As the combusted gas expands in the cylinder from its max temperature at top dead center, the pressure has decreased at 90 degrees of the power stroke because an increase in the volume of the combusted gas decreases the pressure of the gas. At that point, the injected water mist will further...
In a gasoline piston engine, it seems that rather than waste energy out the exhaust, it would be better to inject a tiny bit of water during the expansion phase to produce steam that would raise the pressure against the piston and provide more power with the same amount of fuel.