Not sure which specific example they're referring to, but another example is that, according to tradition, the prophet Isaiah was martyred by placing him inside a hollow tree that was then sawn down. This appears to be referenced by the Apostle Paul in Hebrews 11:37 even though there is no description of Isaiah's death in the Old Testament:
"They were stoned, they were sawn asunder, were tempted, were slain with the sword: they wandered about in sheepskins and goatskins; being destitute, afflicted, tormented;"
"They were stoned, they were sawn asunder, were tempted, were slain with the sword: they wandered about in sheepskins and goatskins; being destitute, afflicted, tormented;"