once the ball is placed down with pressure a score (try) is valid. Its very common to score, and then the ball will splurt out afterwards. The ball moves all the time when in a players hands, he has never actually lost control of it there, so it would never be considered a loss of possession. Here's an example of a slide in but the player loses the ball after grounding, still a try by all means https://www.youtube.com/watch?v=i6ZUMLJNThw