Jet is a material that adds force to other things. It is often used to push things through Wall. When combined with Rigid Axis (As in, not mixed), it makes it spin out of control. Also, with Mochi, it flies away. Try making a Jet liquid (J in Mix Mode) and adding it to Mochi (see Mochi-Jet reaction for more info).
Jet sinks in Water because the impact pushes the Water away and Jet goes into the gap. The Water below the gap is pushed away and Jet sinks deeper.