Documentation:
ArrayExt menu:
"idxSelect" (an on/off switch, or data selector).
This node has a data input and a data output of matching datatype. Said datatype is determined by RMB clicking on the node, and then clicking either "Numbers" for numarray datatype, or "Points" for pointarray datatype (default), or Objects for objectlist datatype. The node also has a controlling numarray input called idx (index). If the value of idx rounds up or down to 0.0 (i.e. -0.5 <= idx < 0.5), then the data input is connected to the data output. (The on-of switch is on.) If idx is 1 (actually if idx <= -0.5, or 0.5 <= idx, then the data input is NOT connected to the data output. (The switch is off).
By varying the value of idx, portions of the data stream wired to the input can be passed on (selected), or blocked.
A rudimentary circle .nod was used to figure out the function.

- Brian

(I made a few minor changes to the nodeeditor Wiki, which is very out of date and incomplete.)
Current directory of extension nodes is difficult to locate...
Nodeeditor is over 3 years old.

I spent considerable time re-reading the parametric design thread, and nodeeditor thread, to try to learn more of how each node works,
and copied relevant portions to a text file, to distill out some documentation