Typically, while the realm had changed, something as simple as just sensing spiritual energy should not provide an increase in strength or capability. Yet that was not the case because what was happening was more than the body simply sensing spiritual energy.
A simplified explanation of it would be that the body was awakening. This was because, while during the body tempering spiritual energy was used to temper the body, the cultivator could not actually feel the spiritual energy, just the effects of it. So when a cultivator entered the Qi gathering realm, the action they performed was only sensing the energy for the first time, but the result was quite a bit more.
