en_tn_lite_do_not_use/2sa/20/08.md

When they were

"When the Joab and the men of Judah"

belt

a strip of leather or other material used to hold clothing or weapons in place

sheathed sword

This means the sword was in its protective covering.

the sword fell

Joab let the sword fall out to fool Amasa into thinking that he (Joab) was unarmed. Now Joab could get very close to Amasa.