Constructor that implements something

Constructor that implements something

I have the code below and I'm trying to understand what it does:
readonly Ido _do;
public Main(): this(new doX()) { }
public Main(Ido do) {
_do = do;
}
What does this do in the first constructor?
this(new doX())
and why the application firstly start by
Main(Ido do)
and then call the
this(new doX())
while I believe it should be other way round