פירוש על עבודה זרה 119:4

If the non-Jew touches the wine in the skin-bottle or causes it to shift in the skin, the wine is prohibited. So if there is a Jew there watching to make sure he doesn’t touch it and the skin is full, then the wine is permitted.

The cask being referred to here is open and therefore, the shaking of the wine does not prohibit it, because the one carrying it would not want it to spill out. Therefore if it is full, the wine is prohibited lest the non-Jew touched it accidentally. If it is not full then the non-Jew probably did not touch it, so it remains permitted.
I should note that R. Papa seems to like coming up with opposite scenarios. In scenario one, a full container is permitted, whereas in scenario two it is prohibited. Rabbis seem to love these puzzles even if the logic is not always so straightforward.