The sub headline (the headline AFTER the headline) is a statement that qualifies the headline and instills a sense of urgency. It is a good place to add a statement like a word or expression that pulls readers further into your copy. The purpose being, if after reading the headline a person is not compelled to read any further, then the sub headline can then pull the target reader into the copy.

Sub headlines can include words that add a sense of urgency, scarcity or curiosity to the headline — such as with those magic words “free,” “guarantee,” “limited” or “deadline.”
“Without a sense of urgency, desire loses its value.”
-Jim Rohn-

However, here’s an important note: How many times have you been given a time-limited or quantity-bound offer, but with no apparent urgency? You probably thought, “I’m sure that offer will be there when I come back.” (And more than likely, you never did go back.) So as long as you justify your sub headline by backing it up with a truthful urgency, you will be more credible, increase desire and pull more people into your copy.