if you forget this, you can inadvertently install stuff to the wrong ruby/gemset/etc.
easy workaround: when you switch to such a branch, close and reopen the terminal window. this will cause rvm to load the ruby listed in .ruby-version
[smr@smr pws2013 (master)]$ git co migrate-to-rails4
Switched to branch 'migrate-to-rails4'
[smr@smr pws2013 (migrate-to-rails4)]$ ll
total 1592
drwxr-xr-x 34 smr staff 1156 Dec 1 17:40 .
drwxr-xr-x 5 smr staff 170 Apr 13 2013 ..
-rw-r--r--@ 1 smr staff 15364 Dec 1 10:30 .DS_Store
drwxr-xr-x 16 smr staff 544 Dec 1 17:40 .git
-rw-r--r-- 1 smr staff 686 Dec 1 17:40 .gitignore
-rw-r--r-- 1 smr staff 14 Mar 25 2013 .rspec
-rw-r--r-- 1 smr staff 8 Dec 1 17:40 .ruby-gemset
-rw-r--r-- 1 smr staff 11 Dec 1 17:40 .ruby-version
-rw-r--r-- 1 smr staff 166 May 22 2013 Capfile
-rw-r--r-- 1 smr staff 2841 Dec 1 17:40 Gemfile
-rw-r--r-- 1 smr staff 7869 Dec 1 17:40 Gemfile.lock
-rw-r--r-- 1 smr staff 3695 Dec 1 17:40 Guardfile
-rw-r--r-- 1 smr staff 9208 Mar 7 2013 README.rdoc
-rw-r--r-- 1 smr staff 272 Mar 7 2013 Rakefile
drwxr-xr-x 10 smr staff 340 Jun 12 2013 app
drwxr-xr-x 5 smr staff 170 Dec 1 17:40 bin
drwxr-xr-x 16 smr staff 544 Dec 1 17:40 config
-rw-r--r-- 1 smr staff 157 Mar 7 2013 config.ru
drwxr-xr-x 132 smr staff 4488 Dec 1 17:40 content
drwxr-xr-x 8 smr staff 272 Dec 1 17:40 db
drwxr-xr-x 3 smr staff 102 Mar 7 2013 doc
-rw-r--r-- 1 smr staff 740672 Sep 30 2013 erd.pdf
drwxr-xr-x 4 smr staff 136 Dec 1 17:40 grs_course_defns
drwxr-xr-x 7 smr staff 238 May 25 2013 lib
drwxr-xr-x 8 smr staff 272 Aug 16 2013 log
drwxr-xr-x 3 smr staff 102 Jul 10 19:10 pdf
drwxr-xr-x 5 smr staff 170 Dec 1 17:40 pdf-templates
drwxr-xr-x 12 smr staff 408 Oct 4 14:38 public
drwxr-xr-x 5 smr staff 170 Sep 23 11:46 script
drwxr-xr-x 8 smr staff 272 Dec 1 17:40 smr
drwxr-xr-x 13 smr staff 442 Dec 1 17:40 spec_old_unused
drwxr-xr-x 8 smr staff 272 Dec 1 17:40 test
drwxr-xr-x 7 smr staff 238 Aug 6 16:46 tmp
drwxr-xr-x 4 smr staff 136 Mar 7 2013 vendor
[smr@smr pws2013 (migrate-to-rails4)]$ rvm current
ruby-1.9.3-p286
[smr@smr pws2013 (migrate-to-rails4)]$ rvm current
ruby-1.9.3-p286
[smr@smr pws2013 (migrate-to-rails4)]$ cd ../pws2013/
[smr@smr pws2013 (migrate-to-rails4)]$ rvm current
ruby-2.2.2@pws2013
[smr@smr pws2013 (migrate-to-rails4)]$ git co master
Switched to branch 'master'
[smr@smr pws2013 (master)]$ rvm current
ruby-2.2.2@pws2013
[smr@smr pws2013 (master)]$ cd ../pws2013/
[smr@smr pws2013 (master)]$ rvm current
ruby-1.9.3-p286