Type 1 surge protection: parallel or series?

Type 1 surge protection devices (SPDs) are installed at the outside edge of electric service, usually between the power meter and the meter base/socket, and protect the entire house. But how do they work electrically?

SPD types 2 and 3 work entirely in parallel with the protected devices; when there is a surge, the SPD basically loads the circuit to draw the voltage back down. Do Type 1 SPDs work this same way, or is there also a series aspect to their operation?

I seem to remember reading that a Type 1 SPD can create some isolation between the house and power line. In other words, it would absorb the surge voltage between (in series with) the line and the house. Is this the case?