Hi Juri. Not sure if I understand correctly what you're trying to do, but you can emit particles only on moving geometry by using the inherited velocity attributes from the RBD in the pop network. If you want, you can cache the RBD object and check if there is velolcity attribute. If there is not, you first have to generate point velocity (you can do this with a trail sop). Then, you can create an attribute at SOP level, call it myvel use type vector, feed the values with velocity $VX, $VY, $VZ. Now in the Birth tab of the source operator inside the POP you can add this expression:
if ($MYVEL > 0, 1, 0) That will emit particles only when the point geometry is moving.
-Alvaro