A flash, also known as a sea-urchin flash, is a kind of speech bubble commonly used in making manga and comics that has lines emanating outwards.
MediBang Pro makes them easy to draw with this handy tool.

Creating

From the Filter, select Flash.
A new Flash will appear on the canvas.
You can use the adjustment panel as seen below to create different kinds of flash bubble to fit your work.

Adjusting

You can also control the length, thickness, and density of the lines to create different effects.