I guess you don't play much Craps or where ever you play is some back woods casino.
I've never seen a string or Rod used on a Dice table, and I have played multiple casinos in Vegas, AC, Bahamas , PR and a couple of countries ... over 22+ Years
It isn't visible, it's under the layout. Look closely and u will see what appears to be a tiny wrinkle going up the width of the table in the middle