• Left Mouse: select a new element; rotate the current element
  • Double Click or Control-Click: expand/collapse this node's children
  • Shift-Click: show/hide this node's attributes, if any
• Middle Mouse: zoom in/out
• Right Mouse: move left/right/up/down
  • Shift-Right Mouse: Rotate entire tree around y- and z-axes.

You can view the Hydra User's Guide in HTML format here, or download the whole thing in PDF format here.