Question about exercise 14.31 in TeXbook

Question

The following example uses \signed macro from page 106 of TeXbook:

\tracingonline=1
\tracingparagraphs=1
\def\signed #1 (#2){{\unskip\nobreak\hfil\penalty50
  \hskip2em\hbox{}\nobreak\hfil\sl#1\/ \rm(#2)
  \parfillskip=0pt \finalhyphendemerits=0 \par}}
\hsize 3.0in \noindent
But sometimes an extra line must be added. \signed N. Bourbaki (Paris)
\bye

Tracing shows:

@firstpass
\tenrm But sometimes an extra line must be added. 
@\penalty via @@0 b=0 p=50 d=2600
@@1: line 1.2 t=2600 -> @@0
 [] \tensl N. 
@ via @@0 b=96 p=0 d=11236
@ via @@1 b=0 p=0 d=100
@@2: line 2.2 t=2700 -> @@1
@@3: line 1.3 t=11236 -> @@0
Bourbaki
@\kern via @@1 b=0 p=0 d=100
@@4: line 2.2 t=2700 -> @@1
 \tenrm (Paris)
@\par via @@1 b=0 p=-10000 d=100
@@5: line 2.2- t=2700 -> @@1

When empty hbox is removed, tracing shows

@firstpass
\tenrm But sometimes an extra line must be added. 
@\penalty via @@0 b=0 p=50 d=2600
@@1: line 1.2 t=2600 -> @@0
  \tensl N. 
@ via @@0 b=96 p=0 d=11236
@@2: line 1.3 t=11236 -> @@0

and then first pass fails. Why?

Answer 1

Glue and penalties are discardable, so without the box the entire

\nobreak\hfil\penalty50\hskip2em\nobreak\hfil

sequence is discardable

If the break happens at the p50 then you would have N Bourbarki (Paris) on the second line, so you get badness from stretching two word spaces to fill the line"

But actually it takes the break after N. as that is penalty 0 which in this case is enough to compensate that Bourbarki (Paris) has only one word space to stretch so more badness from that but no penalty at the break.

Conversely, the \hbox{} is non discardable so if the break is taken at the penalty50, the 2em glue is discarded and the \hbox{} ends up at the start of the next line with the glue expanding to flush the text to the right. (This is basically the difference between \hspace and \hspace* in latex)