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 100mm Sil X's balls are definitely more of a body roller than a juggler; their large surface area gives you loads of control for those tricky contact moves. Of course, you can juggle with them, they're just a bit on the large side.
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.
Warning: The white balls should be stored separately as they can take on the colour of other balls/objects they comes in to contact with.