Evil, anger, tyranny, temptation, obsession, power, the dark side. The chains that bind the figures can represent actual imprisonment, or it can represent shackles of our own making. In many cases the bindings are fragile, but often we are too oppressed or overwhelmed to notice.
This version deviates from the Rider-Waite a bit visually, but I've tried to maintain a sense of the interpretation. Here our crystal ponies are bound to an apparently small thing. It is fragile and hollow, and it appears that they could free themselves with just a little effort. So too our own addictions appear when viewed objectively and dispassionately from the outside - and yet they torment us.