I just found this brush and tried to experiment with its stroke.
I googled it and only found it was mentioned in the 4R8 P1 update.
- A Claytubes-Constant brush has been added. This will allow constant depth to the surface when a morph target is applied.
- Resolved various Brush issues involving Morph Target usage. (Such as the interaction of the ClayTubes Brush with Morph Targets.)
http://www.zbrushcentral.com/showthread.php?207575-ZBrush-4R8-Update-1-Now-Available
http://www.zbrushcentral.com/showthread.php?208273-ZBrush-4R8-Update-2-Now-Available