Figure 15 shows four sketches Kneass drew of steam passing through a nozzle.  In general, compressible flows through a diverging duct increases velocity as a gas expands.  The two sketches at the bottom of figure 15 are both diverging, but the bottom one is slightly curved, and produced the highest velocity flow parallel to the axis.  The area of a duct is proportional to the square of the diameter, and the curvature allows the steam to expand more linearly as it passes through the duct.
An ideal gas cools during adiabatic expansion (without adding heat), releasing less energy than the same gas would during isothermal expansion (constant temperature).  Expansion of steam follows an intermediate thermodynamic process called the Rankine cycle.  Steam does more work than an ideal gas, because steam remains hot during expansion.
The extra heat comes from enthalpy of vaporization, as some of the steam condenses back into dropplets of water intermixed with steam.