How does a jumping spider manage to "jump" on the ceiling?
You are right - most jumping spiders have draglines they secure to the surface before they jump.  They can't defy gravity, but they can recover from a miss if they do this.


  Disturb a jumping spider on a wall, and watch it seemingly defy gravity: it will jump horizontally out into the air, then return to the wall.  This seems fantastic until you spot the major ampullate dragline.  It essentially becomes the bob at the end of a pendulum string. 


the book goes on to explain that the dragline is used as an airbrake and a spring to help the spider control the jump, which might make it seem to do unnatural things when jumping from a ceiling.  You might want to get a video of this if you can. 

Some of the references in another book here, point to other uses of draglines - the spiders can tell each other's species by them, attract a mate.