Can someone explain how T-mobile Jump Works?

Walked into tmobile so I could take a glance at the note 5 and s6+ ( not impressed at all) and blah blah blah ended up talking to a sales person about jump and how it works. I was in a hurry, but she didn't explain the 2 different types very well. I know I'm on the first one where I pay 10$ a month but she mentioned something about them paying off the rest of the phone for me.. but she also threw out there something about them only paying for half of what I owe.. just hoping someone can clear it up. From what I read jump on demand is a leasing type deal. Anyway thanks in advance! !