I'm home from school for labor day weekend and I managed to get in a game against one my friends - who was running his tyranids.
The game was killpoints and I won 8 to 1. Despite this, I still learned quite a few things, mainly when or why you would sacrifice units.
Who makes a good sacrificial unit? Any unit that you have redundancy in or is otherwise not a huge loss when it dies. This can be anywhere from a combat squad of Space marines to Space Marine Scouts to a squad of Guardsmen being led by a Commissar. The consideration you have to make whether or not the unit can easily be broken in combat. I feed my foe Rhinos once my use for them has dwindled. Which leads me to...
When do you want to do this? This is the hardest part to judge - if you know for sure that the unit you feed your opponent will die on his turn, you want to give him the charge or the opportunity to light your guys up. That way, when it rolls over to your turn, you can get some revenge kills in. However, if your unit is tough enough that they can withstand one turn of assault (or he just can't do enough damage to kill all of them in a single turn), then you should charge him on your turn (assuming you have the opportunity). You deny him some attacks, meaning that (hopefully) you'll survive til your opponent's Assault Phase, where he will finish you off and leave himself an open target for your vindication. Now, if you plan on assaulting with your sacrificial unit - you want it to be hard to break. The worst thing that can happen is that your squad gets wiped out or caught in a sweeping advance, allowing your opponent the opportunity to shoot/assault with impunity at your other targets. If you plan on passively feeding him a squad (ie. bait) then make it as easy to kill as possible.
Why? By sacrificing a unit, you can buy yourself time by drawing the fire/assault away from your other units. A common example of this is a squad of kroot hiding in woods and screening the fire warriors behind it. The enemy's assault troops will have to deal with the kroot before they can tear into the fire warriors. Alternatively, by putting a lone (yet somewhat juicy) target away from your other units unsupported, you have a piece of bait that you can use to lure a unit or two away from the main hustle and bustle of the battle.
Those are my 3W's for today. C&C welcome!