From beamer manual:

---
On non-specified slides the \only command simply “throws its argument away”
and the argument does not
occupy any space. This leads to different heights of the text on the first
three slides and on the fourth slide. If
the text is centered vertically, this will cause the text to “wobble” and
thus \uncover should be used. However,
you sometimes wish things to “really disappear” on some slides and then
\only is useful.
---

Rafael, could you describe exactly what you want to happen? When I use
either <1> or \only<1> formats, I see the first block only one slide 1 and
the second block only on slide 2. What is it that's not happening for you?
Or could you paste the LaTeX equivalent that does work so we can see what
it's supposed to do?


Thanks,
John

* Foo
** Slide 1
***  Block1    :B_block:
    :PROPERTIES:
    :BEAMER_envargs: <1>
    :BEAMER_env: block
    :END:
    - bla11
    - bla12
    - bla13

***  Block2     :B_block:
    :PROPERTIES:
    :BEAMER_envargs: <2>
    :BEAMER_env: block
    :END:
    - bla21
    - bla22
    - bla23


On Wed, Sep 29, 2010 at 11:12 AM, Carsten Dominik <carsten.dominik@gmail.com
> wrote:

>
> On Sep 18, 2010, at 8:15 PM, Rafael Calsaverini wrote:
>
>  Hi there.
>>
>> I'm trying to use the \only<> command to produce a beamer presentation but
>> it seems that org-beamer filters out this. I've tried it in many different
>> ways, and didn't get the desired effect.
>>
>> Particularly, I've tried the obviously wrong thing:
>> #+STARTUP:      beamer
>> #+LaTeX_CLASS: beamer
>> #+LaTeX_CLASS_OPTIONS: [smaller, presentation]
>>
>> * Foo
>> ** Slide 1
>> \only<1> {
>> ***  Block1     :B_block:
>>     :PROPERTIES:
>>     :BEAMER_env: block
>>     :END:
>>     - bla11
>>     - bla12
>>     - bla13
>> }
>> \only<2>{
>> ***  Block2     :B_block:
>>     :PROPERTIES:
>>     :BEAMER_env: block
>>     :END:
>>     - bla21
>>     - bla22
>>     - bla23
>> }
>>
>> Which obviously break-up. And I also tried what I believe would be a nice
>> syntax for this:
>>
>> * Foo
>> ** Slide 1
>> ***  Block1    :B_block:
>>     :PROPERTIES:
>>     :BEAMER_envargs: \only<1>
>>     :BEAMER_env: block
>>     :END:
>>     - bla11
>>     - bla12
>>     - bla13
>>
>> ***  Block2     :B_block:
>>     :PROPERTIES:
>>     :BEAMER_envargs: \only<2>
>>     :BEAMER_env: block
>>     :END:
>>     - bla21
>>     - bla22
>>     - bla23
>>
>> Apparently the parser just removes the "\only" and interprets this as <1>
>> and <2>. The resulting latex is exactly the same as if the "\only" wasn't
>> there.
>>
>> Is there a way to use the "\only<>" overlay in a clean way?
>>
>
> What does \only do?
>
> - Carsten
>
>
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