The best way I can explain it in simple terms is that the more wild the exhaust cam, then the faster it lets the gasses out - which means that at low speeds, this happens too quickly for the full ideal burn process to take place. The milder cam retains the charge in the cylinder longer which gives a better burn, which spins the turbo faster. Obviously from this description, you can see that this also has certain trade-offs and compromises once high rpms are reached... Again it's all swings and roundabouts and is about speccing the engine to be compromised in how the customer wants it to drive / it's intended use .