‘And there came over against Gibeah ten eleph chosen men out of all Israel.'
These ten units may have been the liers in wait, or they may have been the forces in ambush that suddenly appeared in front of the horrified Benjaminites, joining forces with the fleeing children of Israel. Or they may have been a third force which had been waiting for this moment. (As often with descriptions, ‘over against' is rather vague although no doubt clear to the writer).
Thus we may read ‘All the men of Israel rose up out of their place --- the liers in wait of Israel broke forth --- and there came over against Gibeah ten units of chosen men', seeing three aspects of the strategy.
‘And the battle was sore, but they knew not that evil was close on them.'
The new strategy had rendered the slingers relatively ineffective for they worked best against massed troops before battle was actually joined, not against rapidly moving fleeing targets, and the retreat had probably disorganised them. The cutting down of fleeing troops was not work for slingers, and the Benjaminites had not been expecting the extra reinforcements.
So now their swordsmen and spearmen found themselves sorely pressed (the slingers may even have joined in the ‘victorious' chase as swordsmen). And they were unaware that worse was to come. They did not know about the liers in wait, and the ten units.