A good example of knowing when to sacrifice your units is this... At the Battle for Salvation tournament, I was facing another Blood Angels player in the final round for the win. He had a Land Raider with terminators inside, 10 vet assalt squad with meltas attached to Mephiston, 8 man Death Company, and corbulo in a razor with a combat squad, all coming at my objective. The mission was bases. My base was defended by my 3 DC, Corbulo, 2 furioso Dreads and my 10 man tactical. Dante was bouncing around. Needless to say, he had a huge advantage. Sure, I had 2 furioso dreads, but he had 18 guys with jump packs, 8 of them Death Company, a Land Raider coming down my throat filled with assault terminators, plus Corbulo coming up behind granting Furious Charge. What I did have was my three Baal Predators. Juicy targets for him! But not all that effective in this case because he would be able to jump behind cover right up until he was ready to assault.
I knew that my only chance would be to seperate his charge out, and try to get his units to come in one at a time. He sent the LR down the middle, the VAS to my left with Mephiston, and the DC to the right, in range of Corbulos razor.
So, I sent one BP to the left to intrcept Mephiston, kept one next to my base, and sent one out to the right. My hope was that he would assault my tanks for a turn or two, then I could attack his forces with my dreads one squad at a time. The first victim up was his Death Company, which could not pass up that juicy Baal Pred. He assaulted, and destroyed it. But on my turn, my Furioso Dreads assaulted and destroyed them! Then consolidated back to my base. Meanwhile, I moved my left Baal Pred into his assault range for Mephiston and the VAS.
On his turn, he could not pass up yet another juicy target! His VAS and Mephiston went for the Baal Pred while his Land Raider and Terminators went for my Tactical Squad. He destroyed my Pred but failed to take out my Tacticals, who then escaped his Terminators since they cannot sweeping advance. (I knew they could not by the way... I was hoping at least one would survive.) On my turn, my surviving tactical ran for cover, while my Dreads moved up and slaughtered his terminators and took out the Land Raider. The key here was that had he jumped his VAS into my tactical squad, he would have won the fight... and in the end, stopped me from winning the game and tournament. But instead, I managed to whittle his large attack force down to individual units, which my two dreadnoughts were able to slaughter one at a time, bu sacrificing juicy targets and splitting up his forces. The Dreads ended up slaughtering his VAS, while my corbulo held off Mephiston for a round. In the aftermath, I had a single tactical marine holding my objective, all of his forces were routed, and Mephiston was running for the hills to keep me from getting the kill point.
You can read that entire batrep somewhere around here.
So, know when to sacrifice units! There is no formula for success. The best use of this is to slow down your opponents charging forces to split them up. But also, at the end of the game when the two of you are working to contest each others objectives, slowing him down by giving him something to kill in turn 4 can mean the difference of inches in his contesting or holding objectives in turn 5.
Just dont be afraid to sacrifice that high point squad! Fritz is a master of this with his Seer Council. All that matters at the end of the game is who is controlling more objectives. He can have 90 percent of his forces left, and you can have 10... but you can still win.