Struct Constructors

[Mark Blume via Digitalmars-d-learn]

Why exactly isn't a constructor without any parameters is not 
allowed?
Why does
"Struct()" calls "Struct.opCall()," which means "Struct.init" 
initially,
while
"Struct(params)" calls "Struct.this(params)?"

Does "Struct(params)" also call "Struct.opCall(params)?"

I am new to D and I can't seem to understand this.