Sketchup::Model#select_tool()says:The select tool is activated if you pass
nilto theselect_toolmethod.
Ex:
Sketchup.active_model.select_tool(nil)
There is also the non-preferred #send_action:
Sketchup.send_action("selectSelectionTool:")
These “actions” are really meant for toolbar buttons or menu item commands, and act asynchronously just as if the user had clicked a button or fired a keyboard shortcut.
So prefer other API methods (when possible) especially if you need a return status from an action.
The #send_action module method only returns a boolean indication of whether it “knows” the requested action on a given platform, and does not block. (Ie, your next code statement could be evaluated before the application finishes doing all it should do for the requested action.)