Katara thought she'd be fast enough to avoid the projectiles: she had an idea of how quickly her teacher was. What she did not expect was for her to dance about the tentacle and then go under it, her foot out. Katara was cut off guard here, realizing here that she'd acted too quickly. Of course, her falling didn't mean she was only one unscathed.
As she fell, she brought her arm back and the pouch water flew again, and she really was lucky Sheeana was crouching: it made it a little easier. The water, once hitting Sheeana's foot, froze it to the ground. Katara, once hitting the snow, reared her feet back and kicked herself up again, back in position.
Re: SNOWBALL FIGHT! Jump on in y'all
As she fell, she brought her arm back and the pouch water flew again, and she really was lucky Sheeana was crouching: it made it a little easier. The water, once hitting Sheeana's foot, froze it to the ground. Katara, once hitting the snow, reared her feet back and kicked herself up again, back in position.