I've been trying to scale some faked rotation on particles using the magnitude of the velocity. The problem is velocity never reaches zero when the particle comes to a rest. Why a particle at rest has a velocity greater than zero is beyond me - I must be missing something about the way houdini solves it's particle update (vs. maya).

Anyway, I've noticed that my pop net produces a ‘backtrack’ variable, a float vector with 4 components that looks suspiciously like the kind of data I'm after (and expect to some degree). What's odd, is this attribute isn't documented anywhere that I can find - any knowledge about this out there? It appears to correspond to velocity, but properly zeros out when the particle comes to rest. What's the 4th component of this all about?
(edit: looks like the 4th component is something like timestep/FPS)

The second part to this confusion is why when I set particles to bounce on collision, with a Normal and Tangent Gain of zero, my particle insists on falling through the ground plane - after a very brief pause due to collision.

Okay, so It makes a bit more sense now (thanks mitch!). The note at the top of the state pop docs says it all, “Most POPs will not process particles with non-zero states, as it would, for example, be inefficient to process dead particles. ”.

So in my particular case, once the particle had stopped, it's state was non-zero and therefore the velocity was not being updated. Adding a velocity pop with an expression if($STUCK,0,$VX) seemed to work around it in my case.

Still don't know why the bounce on collision with values of 0 for gain on normal and tangent causes the particle to fall through the floor, but I suppose you could argue why I would want to do that anyway.

So what is up with the backtrack attr? Plz share.

yeah, I was wondering why you want to set the gain and the normal to 0, when setting it to stop on collision does what you want. It's the normal causing it, not the gain, and I think it has to do with the way that value affects the velocity of the particles after each collision.

As for the backtrack attribute, the first three components are the velocity of the particle from the previous frame.