We've noted that

git checkout testing

moves _HEAD _to point to the testing branch. The significance of this is seen when making another commit

vim my_test.rb

git commit -am 'yet another commit'

in which the branch that HEAD points to moves forward with the commit as shown in the figure below:

           _**See, HEAD is smart enough not to lag behind**_

NOTE: The testing branch has moved forward but the master branch still points to the commit you were on when you ran git checkout to switch branches.