I don't see that she was explaining. She was trying to anser BV's questions in the manner that he was posing them and he was wanting simple yes/no "gotcha" answers to questions that didn not warrant them.
The overriding purpose of Dr. Rice's testimony is to represent -- that is, explain -- the administration's position. I think it's a huge risk with little "reward."
She's doing as well as can be reasonably expected, but I think people are going to use her testimony to bolster their positions however they stand. I just don't see how the president benefits from putting her before this election year charade.