That's the whole point of the flashing. It tells you when it's too late to start walking and make it across the street before the light changes. Some cities have a little quick-reference guide stuck to the pole to remind you of this.

Here in Seattle it used to be quite common to get a ticket for jaywalking. Now it only seems to happen if you block traffic flow on a downtown street during rush hour.

Differs by jurisdiction, obviously, but everywhere I've ever been it's illegal to begin crossing when the light is flashing, and legal to complete a crossing already in progress.