Sil-X stage balls are partially filled with liquid silicon. The liquid centre gives the ball a nice weight and a low centre of gravity making arm/body rolls and stalls easier than standard stage balls.
Foot and neck stalls and head balances are also facilitated by the stability of these balls.
The plug on these balls is fairly discreet and the seam markings are minimal making them reasonable balls to practise isolations but by no means a substitute for an acrylic ball.
The 78mm Sil X's are a great middle ground; they are big enough to be good for body rolling and small enough to be used as stable juggling balls.
The SilX ball still has a little bit of squish to it, making catches nice and comfortable.
Warning: If stored incorrectly these balls can become misshapen. Over time the plug can work its way out of the ball and leak silicone as there is no glue involved with the plug, only friction. Before each juggling session, check that the plugs are inserted flush with the ball. If the ball starts to leak, you need to remove the plug and carefully clean the hole with paper towel then re-insert the plug firmly. This should stop all leakage.
Warning: These balls should be stored separately as they can take on the colour of other balls/objects they comes in to contact with.