If the "jump" coincided with one of the explosions it may have produced a similar effect to a torpedo attack i.e. survivors from torpedo attacks and magazine explosions described how their vessels (including heavily armed battleships) jumped several feet into the air when the explosion occurred inside the ship or from the shockwave underneath the ship which sometimes broke the vessels in two. I wonder if the bow of the Titanic "jumped" because of some underwater explosion or implosion which created a shockwave or bubble inside the ship which forced part of it to jump. When the Titanic broke some survivors said the bow was shaking and reeling, and the stern was trembling and heaving. Sparks and coal were shooting out of the funnels and some said many people were killed by the explosion as both sections tore apart. As the Titanic sinking is unprecedented we really have nothing to compare it with. Often wonder what materials inside the ship could have been involved in the breaking. e.g. Something in the kitchens or stores reacting to a broken pipe or pressures and exploding e.g. Flour or sugar. .