Can't understand the application of delegates in toString() functions

[Heisenberg via Digitalmars-d-learn]

Hi there. I'm currently following Ali Çehreli's "Programming in 
D" book and I can't seem to be able to wrap my head around the of 
delegates in the toString() functions..

http://ddili.org/ders/d.en/lambda.html
(Bottom of the page)

> We have defined many toString() functions up to this point in 
> the book to represent objects as strings. Those toString() 
> definitions all returned a string without taking any 
> parameters. As noted by the comment lines below, structs and 
> classes took advantage of toString() functions of their 
> respective members by simply passing those members to format():

> ... code ...

> In order for polygon to be sent to the output as a string on 
> the last line of the program, all of the toString() functions 
> of Polygon, ColoredPoint, Color, and Point are called 
> indirectly, creating a total of 10 strings in the process. Note 
> that the strings that are constructed and returned by the 
> lower-level functions are used only once by the respective 
> higher-level function that called them.

Okay, I get it.

> However, although a total of 10 strings get constructed, only 
> the very last one is printed to the output:
>
> [{RGB:10,10,10;(1,1)}, {RGB:20,20,20;(2,2)}, 
> {RGB:30,30,30;(3,3)}]
>
> However practical, this method may degrade the performance of 
> the program because of the many string objects that are 
> constructed and promptly thrown away.

I can see the reasoning behind this. But ..

> However practical, this method may degrade the performance of 
> the program because of the many string objects that are 
> constructed and promptly thrown away.
>
> An overload of toString() avoids this performance issue by 
> taking a delegate parameter:
>
> CODE: void toString(void delegate(const(char)[]) sink) const;
>
> As seen in its declaration, this overload of toString() does 
> not return a string. Instead, the characters that are going to 
> be printed are passed to its delegate parameter. It is the 
> responsibility of the delegate to append those characters to 
> the single string that is going to be printed to the output.
>
> All the programmer needs to do differently is to call 
> std.format.formattedWrite instead of std.string.format and pass 
> the delegate parameter as its first parameter:

Why? How can a delegate which returns nothing be used as an array 
which is going to be printed on the screen? And just how is it 
printed? The Documentation didn't make it much clearer.. the 
'sink' is supposed to be the 'writer', isn't it? So what does it 
do? The arguments get interpreted and formatted into the string, 
but what happens after that?

P.S. I've just checked the book, it doesn't seem to be explained 
anywhere properly (the way formattedRead is)
P.P.S. I'm following the PDF version of the book