When you stick your neck out to make a bid, you are aware you might get doubled and go for a big penalty (if partner has the wrong hand). So the upside has to be greater than this downside.

The main upside to making a bid on a light hand that does not expect to win the auction is to attract the best opening lead from partner. So, simply, the more you want partner to lead your suit, the more you should stick your neck out.

Would you stick your neck out after (1 ♥ ) - P - (1 ♠ ) - ? with the following hands (say Neither Vul):
 

Hand i	Hand ii
♠ K Q 7 ♥ J 9 ♦ J 9 7 4 3 2 ♣ A 6	♠ K J 6 2 ♥ 9 6 2 ♦ K Q 10 9 3 ♣ 8

• Pass with (A); 
• bid 2 ♦ with (B). 
• The opponents are quite likely to end up in a Heart contract with partner on lead. In (A) you do not want a diamond lead; 
• in (B) you do – much more than his likely alternative of a club (the other unbid suit).