Hello there,
A switch works by connecting one lead to another lead more or less. But once that lead is connected to the other lead, the voltage goes to zero. So there is no voltage left to power anything. At first though we do have voltage. In a 120vac system we would have 120vac which has 170v peak, and that's plenty of voltage, at least for fairly low current. But once that switch is closed the voltage goes to zero because the idea is to supply all the voltage to the bulb and not to loose any.
So the voltage goes away very completely when the switch is closed. But that's a switch. A triac has a small voltage drop. So one possibility is to use a small transformer to step that voltage up, but that might be hard to do because when the switch is off we would need to disconnect the transformer.
It's the voltage that goes away, but interestingly that's when the current starts. The switch closes or the triac turns on, the voltage goes low, but the current then goes high. Since we need power even with the switch closed, one way to get that power would be to use a current transformer.
The current transformer would tap energy from the flow of the current and supply a small light current power supply. So when current is flowing and the light bulb is on, we still have power. When the switch is off we have plenty of voltage so that's not a problem. So you'd have to wire "OR" the two power sources together. The first source is when the switch is open, the second source when the switch is closed. Rectifier diodes would perform the wired "OR" function.