johnu, on 2021-January-08, 01:21, said:
If you've looked at some of the hands from Nicolas Hammond's Suspicious Leads project, you'll see that some players don't need signals from partner because they already know what partner has before they lead 

Funnily enough I was thinking about that earlier in the thread. If you do not play Journalist or similar for the J/T/9 leads then partner will often have a guess from a holding like Axx. By signalling on opening lead (eg by a vertical/horizontal lead) you can get the benefits of avoiding such a guess and much, much more without having to give up the information to Declarer. Since the weaker lead from hands like JT9x(x), JT8x(x) and T9x(x) is comparatively rare, this would be extremely difficult to detect from normal means.