Pretend you are below the Hoover damn and want a drink of water. Do you want the damn to break and all the water to rush out at once or do you pick up the garden hose and turn it on just a little bit.
Very large amps, especially tube amps require a large amount of current when first turned on. This is due to devices in the power supply which store energy. When these capacitors have reached a fully charged state the demand for current (your drink of water) subsides.
Many modern amps have a circuit which does this, some do not, your P10 has such a circuit also, to ensure the damn doesn't burst when you power-up your system.
