If you kick someone in the backside, you are forcing them to action. You would usually use this phrase when you are referring to making someone who is normally lazy or slow-moving do what they are supposed to do.
"They are so lazy; how did you get them to finish on time?"
"I had to kick them in the backside, but they got it done."
Other words you may hear used in the place of backside:
Student or Learner