My advice remains to put safety ahead of cost. "Just good enough" is fine for lots of things, including wrenches, but not where your health and well being are concerned, IMO. A chain saw can kill or maim you in a hurry. Inertial chain brakes are standard these days on most decent saws. I have a 1998 Husky 55 and it has an inertial brake and my guess is that a model 51 would have one as well. What they do is stop the chain when the tip of the bar receives a shock, such as that caused by a solid kickback. All saws have regular, break-your-hand-to-stop-the-chain brakes as well. If you take the sidecover off, you should see a stainless steel band around the clutch housing. When the brake trips, it squeezes that piece of ss around the clutch drum, stopping the chain immediately. The best way to check if your saw has an inertial chain brake is to push the manual chain brake lever forward. If it clicks and the chain won't move, then the inertial brake is working. Pull the lever back towards you and it should click back into position, and the chain should move freely. You can also check your brake by holding the saw with the bar tip located over a stump or chunk of wood. Let go of the front handle and let the bar tip hit the wood. The chain brake should engage from the shock, and require you to pull the lever forward to unlock the brake.