Start transaction not working with Revit 2014

You're right, it's not being used correctly.

The Transaction needs to take place inside the Idling event.

The button click handler and Idling event handler should look something like this in Revit :

void revitApp_Idling(object sender, Autodesk.Revit.UI.Events.IdlingEventArgs e)
{
UIApplication uiapp = sender as UIApplication;

UIDocument uidoc = uiapp.ActiveUIDocument;

ElementSet elems = uidoc.Selection.Elements;

if (elems != null)
{
label1.Text = elems.Size.ToString() + " items selected.";
}
else
{
label1.Text = "No elements selected.";
}

if (shouldRun)
{
using (Transaction trans = new Transaction(uidoc.Document, "Hide elements"))
{
trans.Start();
uidoc.Document.ActiveView.HideElements((from Element el in elems select el.Id).ToList());
uidoc.RefreshActiveView();
trans.Commit();
}

shouldRun = false;
}
}

private void button1_Click(object sender, EventArgs e)
{
shouldRun = true;
}