Brewer’s Dictionary of Phrase and Fable says, in the entry for Yoke, “To pass under the yoke. To suffer the disgrace of a vanquished army. The Romans made a yoke of three spears—two upright and one resting on them. When an army was vanquished [i.e. beaten], the soldiers had to lay down their arms [i.e. weapons] and pass under this archway of spears.”
The illustration shows three vertical spears and one horizontal. The conquored soldiers appear to be encouraged to pass under the horizontal spear by a man brandishing a long dagger or possibly a whip. The losers have a cloth wrapped round their waist, but are bare-backed, bare-legged and either barefoot or wearing light shoes or sandals. It might be that they were subsequently taken away and enslaved or killed, or if they were mercenaries I imagine that they might even have been re-hired by the winning army. I didn’t find any description of this image in the book, though.