gem install piston

First, you need to import the remote repository location:
example:

 $ piston import http://dev.rubyonrails.org/svn/rails/trunk vendor/rails
 Exported r4720 from 'http://dev.rubyonrails.org/svn/rails/trunk' to 'vendor/rails'

 $ svn commit -m "Importing local copy of Rails"

When you want to get the latest changes from the remote repository location:

 $ piston update vendor/rails
 Updated 'vendor/rails' to r4720.

 $ svn commit -m "Updates vendor/rails to the latest revision"

You can prevent a local Piston-managed folder from updating by using the lock subcommand:

 $ piston lock vendor/rails
 'vendor/rails' locked at r4720.

When you want to update again, you unlock:

 $ piston unlock vendor/rails
 'vendor/rails' unlocked.

If the branch you are following moves, you should use the switch subcommand:

 $ piston import http://dev.rubyonrails.org/svn/rails/branches/1-2-pre-release vendor/rails
 $ svn commit vendor/rails

Vendor branch is renamed, let's follow it

 $ piston switch http://dev.rubyonrails.org/svn/rails/branches/1-2-stable vendor/rails