On Demand Consulting
Author: Mikhail Zilbergleyt

First, we need to determine which elements from TWList were selected.

Based on the Java Script API reference article in the Knowledge Center:
int[] listAllSelectedIndices Returns an array of all of the selected indices of this array. The indices are returned ordered so that the most recently selected index is in position 0 and the least recently selected at the end.

Because the internal JavaScript implementation is different for Client Side Human Services and Server Side I will address them separately.

Server Side Processing ( e.g Heritage Human Service )
Although the KC states that listAllSelectedIndices is an int[] on the server side it is not the same as the native JavaScript Array.
listAllSelectedIndices is dynamically generated from the listSelected TWList which stores all selected list elements.

In my examples the tw.local.mylist variable is declared as List of Strings.

One possible implementation iterates backwards over sorted listAllSelectedIndices and removes elements from tw.local.mylist:
 
On the Client Side Human Service the listAllSelectedIndices and tw.local.mylist are a native Java Script array.
The listSelected  is not available on the CSHS.

Native java script array has no method to remove an array element.  Thus we need to copy all not selected elements into new array:

var purged = []
var selected =  tw.local.mylist.listAllSelectedIndices;
for ( var i=0; i<tw.local.mylist.length; i++ )
{
   if ( selected.indexOf(i) == -1)
       purged[purged.length] = tw.local.mylist[i];
}

 tw.local.mylist = purged;