This 'I think they're going down' double reserved for (i) an opposing 1NT opener, (ii) opposing bids after partner has bid and (iii) clearly the end of an auction, in doubler cannot conceivably wish to bid. Otherwise doubles of suit bids are for take-out, if partner has not made a positive bid.

Exercise: Which of these doubles are for pernalty and which for take-out?
   

a)	S W N E 1♠ P 2♠ Dbl
b)	S W N E 1♠ 3♠ P P 2♠ 4♠ P Dbl
c)	S W N E 1NT Dbl
d)	S W N E 1♥ P 1NT Dbl

   
a) East is doubling a suit bid with West having not made a positive bid. East has short spades and wants his partner to bid.

b) East's double is penalty. It is dearly the end of the auction, inconceivable that East, who did not utter over 2 ♠, could wish his partner to bid over 4 ♠. He has some reason for thinking the opponents have overreached: perhaps a trump holding of ♠ QJ109 (but not a finesseable ♠ Q1086).

c) Being the double of a INT opener, West's double is for penalty, showing (in this situation) any hand with 16+ points (balanced or unbalanced).

d) Take-out - East is not doubling a lNT opener. This double shows the same hand-type as the double of a 1 ♥ opener: ie three-suited opening values with short hearts.