I'm not quite sure what the first part of your post is about. Where do you think the different emphasis comes in those two sentences?
In "He jumped in", it's simply the preposition half of the phrasal verb "to jump in".
In future posts, please don't ask two unrelated questions in the same thread. It can get very confusing when people start to answer only one of the questions. One question per thread please.
Student or Learner