Regular air contains 21% oxygen and the rest is basically nitrogen. That percentage of nitrogen causes an anesthetic effect at depths greater than about 100 ft. that reduces mental ability and coordination.
With trimix, helium is used to replace part of the nitrogen. Since helium has less of an anesthetizing effect than nitrogen, divers can go much deeper and still have an equivalent nitrogen depth of less than 100'. That means they can go to 200 or 300 feet and have the same clarity of thought and coordination they would have at 100 ft or less. That provides a great increase in safety during the dive.
There is no free ride however. Helium is a "faster" gas that is absorbed and released faster than nitrogen so there are some differences in decompression schedules and ascent rates are more critical. Decompression stops tend to be deeper and it's very common to use higher percentage O2 mixes (nitrox) to speed decompression. Helium also causes divers to lose more heat through respiration so good thermal protection is even more critical.
On very deep dives, (below about 190 to 240 ft feet) oxygen toxicity begins to become a problem with mixes with "normal" 21% oxygen, so the oxygen is reduced for mixes used on deeper dives to avoid problems with toxicity. If the O2 percentage is below about 16%, it is not enough to support a diver at the surface, so divers need to use a "travel mix" until they get to 60-70 feet underwater to ensure they get enough oxygen. So trimix diving gets more complex and it becomes very critical that divers follow very careful procedures in identifying and switching to the right gas mix at each point in the dive. That is a major focus of technical training as one of the biggest potential killers of technical divers is switching to a gas with too much O2 in it.
But, if the BF is an experienced diver, is careful and is not generally absent minded to begin with, he will do fine with trimix diving. And if he plans to explore deeper wrecks, trimix will let him do it with a much higher degree of safety.
