Don't forget that the first steam engines (ignoring Hero' primiitive steam turbine of Roman days) were condensing engines. Steam entered a cooled cylinder and the piston was sucked down as the steam turned back to water.

In simple and compound engines the "cut-off" determines how far the piston moves purely from the steam pressure direct from the boiler and how much of the piston stroke is using the power of that amount is steam to expand, reducing pressure as it does so.

Compounding moves it on to another cylinder to use the expansion further. The ratio of cylinder sizes has a big effect on the efficiency, bearing in mind that a variation in cut-off will affect this as well.

Robin