This is an old trick on how to increase the stability of a lightweight tripod : just add some weight under the center column.
To see the effectiveness do the bump test : tap the tripod with and without the extra weight. The quicker it stops wobbling, the better the stability. (this is a good test to perform when you buy a tripod...)
BTW try not to use the center column extended and if using a multi segment tripod like mine, leave the thinnest section in .
Franco