Here we'll do a step by step setup of a simple PFlow particle system in 3D Studio Max, so we'll get the following:

Download max file 

We see green cubes spawned upwards in a vortex motion, then they pop into several little red boxes that fall back down again and stay on the floor. For illustration purposes the boxes hitting the ground turn blue, we'll see later on why this is relevant. 

To create a PFlow system: Menu Create → Particles → Particle Flow Source. Then Drag the PFlow Icon open with the mouse in top view. For our example, rotate the Icon by 180 degrees so the arrow points upward. 

The Particle View for this system looks like this (I added the numbers so we can have a look at each operator and what it does):

To access the Particle View press "6" or select the PFlow icon and press "Particle View" in the modify pannel. Operators can be moved to events from the depot in the lower section of the screen by drag and drop. See the operator's properties by selcting them. Delete, copy & paste, turn on and off operators by right click on the operator. New events are created by dragging an operator on an empty space. Linking events is done by left-click dragging from handle to handle.

1: In the Birth operator we define how the particles are created. Here, 100 particles are emitted over the timespan from frame 0 to frame 60.

2: The Position operator defines where the particles are created. In our case, the location is set to "surface", so they will be coming from the whole area of the PFlow icon.

3: The Speed operator doesnt just set the speed of the particles, "speed" also includes the direction of travel in PFlow. In our example we have a speed value of 105 with a variation of 8.55 (so not all the particles will leave the point of origin at the same velocity). The direction is set to "Along Icon Arrow", so the particles will travel in the direction the icons arrow points. However, the divergence is set to 15.5, so we have a little bit of variation here, too.

4: To add a little bit of chaos to the whole thing, we add a Spin operator and let the cubes tumble around a bit. We set them here to spin around the axis of their travel trajectory (Speed Space). A little bit of variation in the spin rate adds to the percepted "chaos", too.

5: Until now the particles were just traveling straight in their initial direction, but we want them to twirl around themselfes like in a tornado. So we add a vortex spacewarp to our scene and add a Force operator to our pflow event. There we add our newly created spacewarp to the list and adjust it's influence until we're happy with the motion.

To create a vortex spacewarp: menu create → SpaceWarps → Forces → Vortex. Then Drag the Vortex Icon open with the mouse in top view. For our example, rotate the Icon by 180 degrees so the thin arrow points upward. Place the icon so it's in the same spot as the PFlow icon. Scale the Vortex Icon so its circular arrow fits around the PFlow Icon with some space between the two.

6: The Shape operator determines what the particles look like, here we select "cube".

7: The Display operator determines how the particles are presented in the viewport. Because we don't have that many particles, we can set it to "geometry", so we can see what it all will look like in the final rendering. Here we also give the cubes a nice green and we're almost done with the first event.

8: Since we want to "pop" our green boxes at some point, we finally add a Particle Age test operator. The test operators will perform constant tests on the particles and send them along to the next event it is wired to when the test results are "true". As the name already states, this operator will test the particles age, so we set it up to a  test value of 108 frames with a variation of 50 frames. So PFlow will now assign a "deadline age" for each particle randomly between 58 frames and 158 frames, and will send them along to the next event (Red Boxes in our example) when that age is reached.

9: In our event "red boxes" we want the green boxes to spawn a lot of red boxes, so we first include a Spawn test operator. Here we set it to "once" ( because we want just one "outburst" of red boxes per green box) and set the offspring number to 5, so every green box will spawn 5 red boxes. We set the speed to "inherited" (from their green-box parents) and muliyply it  by setting the value to 264%. Additionally we give it some variation (so the red boxes burst out at different speeds). We also give the new boxes some divergence so that the 5 new boxes per parent differ in direction (remember that "speed" in PFlow means velocity & direction!).

As you might have noticed, we actually get 6 red boxes per green box, not 5 as we stated. This is due to the fact that we left "Delete Parent" unchecked, so the parent green box particle stays and gets through the "Red Boxes" event, thus becoming red (12) and smaller (11) just like the new red paricles. If we check "Delete Parent", the "old" particles get deleted at the spawn test operator and we only get to see their 5 offsprings each.

10: Now the new boxes need to fall back down, so we add a gravity spacewarp to your scene, add the Force operator to the event in Particle View and assign the newly created gravity spacewarp (just like the vortex earlier) to it. If you want the red boxes to be affected by the vortex too, just add the vortex spacewarp to the list.

To create a gravity spacewarp: menu create → SpaceWarps → Forces → Gravity. Then Drag the Gravity Icon open with the mouse in top view. Make sure the arrow points downward.

11: We again add the Shape operator set up to cube-particles, but here we make 'em smaller.

12: Still we dont have that many boxes, so we can set the Display operator to "geometry" again. This time we make the boxes red though.

13: We now need the red boxes to land on the ground, so we create a standard deflector plane in our scene. We then add a Collision test operator to our event, and add the deflector to the list. We set the operator to "Test true if particle collides" and set the speed to "stop", so the particles will do just that: when they collide with the deflector, they stop.

To create a deflector: menu create → SpaceWarps → Deflectors → Deflector. Then Drag the Deflector plane open with the mouse in top view. Make sure it's big enough so all falling red boxes will hit it. In our example i added a plane object in the scene to represent the ground, if you do this make sure the deflector and the plane object match.

We'd be done here, however, we have a little problem: the particles still have spin from the spin operator in the "Green Boxes" event, so they wiggle around laying there, which isnt what we want. To solve this problem, we create another event and link it to the collision operator, so the particles that have colided and stoped (velocity and direction) will be sent to the next event, where we get rid of the spin.

14: So here we just add a Spin operator and set the spin rate to 0. Like that the boxes laying on the ground don't wiggle around anymore.

15: We want to see in the viewport what's happening, so again we add a Display operator and set it to "geometry". Just to illustrate that we are in a new event here, i turn the particles blue.

 And we're done!