Anotyher place for prefernce  signal  as  Rusty  explauined  in  13 

is when you know your partner is going to ruff   the size of the card  tells him what to lead back for another ruff     Say you have the ace of hearts... Spades are trump

  and you know your parnter is  trumping  diamonds    You hold  J 3  so you lead back the  J     This is one  I tend to  forget   to think about in my haste to give  partner a ruff     I see many advancers   take the ace of  Heart  before sending the   card to ruff