Another 'Expert Advisor Sample' question

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In the code below taken from the 'Expert Advisor Sample' post on this forum:

-- Buy
if(MacdCurrent<0 && MacdCurrent>SignalCurrent && MacdPrevious<SignalPrevious &&
MathAbs(MacdCurrent)>(MACDOpenLevel*Point) && MaCurrent>MaPrevious)

-- Sell
if(MacdCurrent>0 && MacdCurrent<SignalCurrent && MacdPrevious>SignalPrevious &&
MacdCurrent>(MACDOpenLevel*Point) && MaCurrent<MaPrevious)

What is the significance or purpose of comparing the MACD Main and signal line to each other? I thought the whole point behind MACD was comparing it to the signal line.

Thanks in advance,

Ed