Use two hands!! One of the most basic things to start doing great body work, is to hold onto the sanding block properly; with both hands, whether you’re using True Blox, or any other block. This is especially true with True Blox, since they need to flex to create the proper shape with the panel; which can’t happen with one hand.
Of course, there’s certain situations where both hands don’t ‘need’ to be used. Also, wet sanding is not included in this. (Although I use both hands quite a bit them too)
Besides the block not working right, it never looks like someone is overly concerned about the quality of their work when they are block sanding with one hand.