What does Helen do with Belle in The Miracle Worker?

Belle is an Irish Setter, a beloved member of the Keller
family.  Although she is not a character usually focused upon in The Miracle
Worker, I applaud you for asking this question in order to highlight such a
special minor character here!

Let's take a look at one of
the first instances when we see Belle with Helen (apart from when Belle is simply
sleeping by the pump).  In the stage directions on page 31 it
states:

readability="8">

In the house the setter Belle flees to
the family room, pursued by Helen with groping hands; the dog doubles back out the same
door, ...

From
this, we can infer that Helen gives affection (probably very rough affection) to Belle
whenever Belle is willing to take it. In fact, this is confirmed when we see Helen
interact with Belle near the end of the two-week hiatus from the
family:

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When [Helen] encounter's Belle, she
throws her arms around the dog's neck in
delight.

Kate:  Doesn't she need affection too,
Miss Annie?

Now, these two
instances are fine and good, but the instance that I think you are referring to is a bit
later in that very scene around page 99.  Annie is desperately trying to explain to the
Kellers why she needs more time with Helen and exactly why their affection for her could
be to her detrement.  In the middle of the conversation, Helen gets their
attention:

readability="18">

Helen is playing with Belle's claws;
she makes letters with her fingers, shows them to Belle, waits with her palm, then
manipulates the dog's claws. ...

Keller: 
Teaching a dog to spell

(A
pause.)

The dog doesn't know what she means, any
more than she knows what you mean, Miss sullivan.  I think you ask too much, of her and
yourself.  God may not have meant Helen to have the--eyes you speak
of.

Perhaps more important is
how Annie manipulates the situation, gently chiding
Helen:

Annie
[Gently]: No.

(She shakes her head, with Helen's hand to
her face, then spells.)

Dog. D,o,g.
Dog.

(She touches Helen's hand to Belle.  Helen dutifully
pats the dog's head, and resumes spelling to its paw.)

Not
water.

At this point Annie
gets some water for Helen, trying to make her understand.  However, Helen continues to
try to spell water to the dog and even tries to "thrust Belle's paw into
it."

It is safe to assume, then, that although
Belle become's part of Helen's instruction, Belle is not the instrument of enlightenment
as the water from the pump is.  Just as the family (too in love with Helen herself to
properly help) can never be Annie (the true teacher).  Belle (the beloved family pet)
can never live up to the sensory experience of water (the item burned into her memory
from an early age through sight, sound, touch, taste, and smell):  the only word Helen
had ever spoken, ... and the first word she will speak again.