\action<visible@2>{text}
\action<uncover@2>{text}

- "text" becomes visible on the second slide of the frame only
- difference between "visible" and "uncover" only if \setbeamercovered{transparent}
  command is used


\action<uncover@2->{text2}
\action<uncover@3->{text3}
\action<uncover@4->{text3}

- the specified texts become visible from the second, third, fourth slide of the frame on

the above is the usage of the following structure/command:

\begin{actionenv}<action_specification>
environment_contents 
\end{actionenv}

This environment extracts all actions from the "action_specification" for 
the current mode. For each action of the form "action@slide_numbers", 
it inserts the following text: 
\begin{actionenv}<slide_number> at the beginning of the environment 
and the text \end{actionien} at the end. If there are several action 
specifications, several environments are opened (and closed in the appropriate order).

short form:

\action<action specification>{text}

- this has the same effect as putting text in an actionenv.

----------------------------------------------------------------
Example 1:

\begin{frame}
  \begin{actionenv}<2-| alert@3-4,6>
  This text is shown the same way as the text below.
  \end{actionenv}

  \begin{uncoverenv}<2->
    \begin{alertenv}<3-4,6>
    This text is shown the same way as the text above.
    \end{alertenv}
  \end{uncoverenv}
\end{frame}

Example 2: 

\action<alert@2>{Could also have used \alert<2>{}.}