With his previous move, he'd left his cavalry unit undefended, in full line of attack of Bookthorne's spearman. It would be an easy enough piece for Bookthorne to capture, but what he feared was what was to come after it. Oliver's other cavalry piece was certain to go on a rampage.
And so, to the disbelief of the crowd, Bookthorne was forced to let Oliver's dangerously placed cavalry unit live, in the efforts of defending his position in other places. He reoriented one of his spear units, so that its defence was more focused on the flank than it was on going forward.
"Hoh, there's a mistake," Karstly noted, from amongst the crowd, his amused smile growing by the second.