I had previously thought this was obvious, however, I have had to whip out the 'ol rulebook on multiple occasions to prove this works, even against skilled players, so do not attempt this without a rulebook containing pages 76 and 28 or a signed note from a large and intimidating male who says this is okay to do.
You see, penetrating hits caused against vehicles count as 2 wounds and glancing hits count as 1 for the purposes of combat resolution. This is so you can figure out if you've beaten a walker or a chariot in combat or not. Just the same, it lets you rack up silly amounts of points for your side when determining the outcome.
Below, we see some Screamers who have moved up to confront a squad of Slaanesh Chaos Space Marines in various states of completion because I have a PS3, an Xbox 360, a Wii, two kids, an attractive wife and a short attention span. Now then, look at that Rhino sitting next to them. It is a pussy. Let's go beat its ass.
If you remember your rules for disorganized charges, you will know right away that your primary target for this charge must be the Rhino because if the Marines are the primary target, I have to move models into BTB with an unengaged model in the primary target before I can attack the secondary.
So that's what I did here. I sent five Screamers at the Rhino and a mere two at the CSMs. As these are Slaanesh Marines, they are likely to kill one or two of my Screamers before they get to swing. Too bad. I'm sending 15 attacks at that Rhino so's I can beat the living tar out of it. That should be enough to cause somewhere in the neighborhood of ten penetrating hits. On page 28 of dem rulesbooks it says that you total up all the wounds caused by each side and that all units on the losing side must check morale with all the same negative modifiers. This gives me a score of 20-4, or so. As these are I5 Slaanesh Marines, they have a 50/50 shot of getting swept.
Now, ideally, I prefer to stay in combat through my opponent's turn, but there are situations where this is advantageous to do. This past weekend I used this technique against a Rhino and three Attack Bikes. The Bikes didn't suffer a wound, but I pounded the Rhino so decisively that the Bikes were utterly demoralized and ran off the board to go cry about it. I find it's a better move to use later in the game when there are fewer threats on the board to shoot at you, and you want to destroy as much as possible to either rack up kill points or prevent speedy things like Attack Bikes from scooting over and blasting you off objectives.
Against fearless opponents, this is a useful way to pull punches so you can stay in combat through your opponent's turn. The trick is calculating in your head exactly how many attacks to throw at the vehicle and how many to throw at the unit to both destroy the vehicle and allow you to wipe out the foe in the following turn.