During heads up play, the button pays the small blind and acts first pre-flop.
Post flop and succeeding rounds during heads up play, the button position is last to act.
There is an obvious advantage acting last in any betting round and it is no different in heads up play; the small
blind has to call pre-flop to meet the big blind or even make a raise, but can use the table position of acting
last post flop to decide how or if to play.
It may be easier to understand why the small blind is the button by looking at it, as if the other way round
(the button was also the big blind).
The heads up game would be very different if the big blind was also the dealer, because they would have the major
share of the chips already invested in the pot, and the advantage of acting last both pre and post flop.
With the balance tipped so much in favour of the button, if you were the small blind you would want a much bigger
hand to justify playing when the big blind had already invested chips and would have position for all of the hand.
