Is the signal from the receiver to the extra amp and amplified signal, or is it like the preout to a subwoofer-basically not amplified?

The reason I ask is if the wattage is "stacked"-if my receiver is putting out 115 wpc and the external is 100 wpc, does that mean 215 wpc are going to my speakers, or is the receiver's amp bypassed and only the external amp handling the signal?