This sentence expresses her feelings.

The rabbit (she) doesn't understand why the tortoise (a person) is so slow. I don't know her heart.

As for "handstand", I think it may be a mantra or code word that she and her friends only know.

I don't know what it is yet. I guess it means "dizzy".

If you really want to know what this means, you'd better ask her friends and others quietly.