The xpTendril modifier, like the xpBranch and xpSpawn modifiers, generates additional particles (though unlike xpSpawn, it doesn't need a separate emitter).
You can think of the various particles as either being source particles (the ones generated independently by the emitter) or tendril particles (the extra ones generated by the modifier to create a tendril).
For each source particle, the modifier generates a stream of tendril particles which start at the same point as the source and follow it.
They can either follow it slavishly (if the Follow Source parameter is enabled) or move independently, but in the same general direction.
It is crucial to understand that the modifier is not producing a series of static particles.
It is generating a constant stream of them, each of which lasts as long as the Tendril Length setting and are then replaced with a new one.
They can all move independently (if Follow Source is disabled) so they can be influenced by modifiers in the scene.
For example, if used with the xpTurbulence modifier, this gives a wavelike, undulating stream of particles.
In this scene, xpTendril is being combined with the modifiers, xpTrail, xpSplineMesher and xpConstraints, to create three moving tendrils.
xpTendril Object tab menu.