I am trying to understand how PT flag actually works
I have gone through this page
http://httpd.apache.org/docs/2.4/rewrite/flags.html#flag_pt
and http://borkweb.com/story/apache-rewrite-cheatsheet
As I observe , specially as per later url where example is like

RewriteRule ^/abc(.*) /def$1 [PT]
Alias /def /ghi

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I modified 2 things in the example
1. removing / forward slash from pattern in the first line as / lets not match pattern with url
2. changed Alias with Redirect or Redirectmatch

so my .htaccess file located at my local server under Applications/AMPPS/www/h/ or http://localhost/h 
looks like :

RewriteRule ^abc /h/def [PT]
Redirect /h/def /ghi

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with or without PT , I got same results( finally redirects to http://localhost/ghi which is pretty expected ).
same with another example i tried, which is something like :

Redirect /h/icons /hello
RewriteRule pics/(.+)\.jpg$ /h/icons/$1.jpg

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that finally redirects to http://localhost/hello 

so I am not getting any idea how PT makes a difference while talking in perspective of .htaccess file, mod_rewrite and mod_alias.