Maximally exploitive strategy question

[SeanC]

Hello,

I'm reading Mathematics of Poker and I've hit a snag with one of the examples of determining the maximally exploitive strategy. To paraphrase, the situation is as follows:

Pot has 3 units. Play A has a flush draw that he gets one last (closed) card to draw to. He will make the flush 20% of the time. B has two-pair. A will bet his flush always and will bluff a certain percentage of the time based on the pot. We need to determine B's MES.

The book says B's MES is to fold all the time if A bluffs less than 5%, obtaining an ex-showdown equity of 3x, where x is A's bluffing frequency.

My first question is why is the pot size * the bluffing frequency the equity of the strategy in the case of bluffing less than 5% of the time?

Next it says that if A is bluffing more than 5% of the time, B's MES switches to calling all the time. Why? The expectation of this call is given as x-.2. Why does this expectation function change and how is it determined that it changes at 5%?

MOP has been very good at fully explaining everything else so far, but I'm having trouble with this one.

Any help is appreciated.