[Psionic Powers] With an Expanded PK Shield, when do rolls occur?

[Mercurae]

I've scoured books and the forum but I could not figure out exactly how this is supposed to work. PK Shield in Psionic Powers says

Every time you are hit by a physical attack, roll against your skill

The Expansion Technique says

You can use your ability to protect others, though this halves your effective level (round down). Success expands your coverage to a two-yard radius for a minute. Double this radius, cumulatively, for every 5 points by which you make your (modified) skill roll.

Does this mean you roll at -5 to activate the expanded shield for 1 minute then roll unmodified every time something in the radius (like an ally) gets hit by an attack? This seems the most reasonable but it isn't very clear. When does the Expansion roll occur? Is it activated by an action or can it be used as a reaction to an ally getting hit?