Let's say you have a power and you want to do something a little different with it. A canonical example would be a hero who had a Dazzle power deciding to use it for Blast just this once.
The way you do that is by use of Extra Effort (Page 120 in the Core Book). In fact, you can use Extra Effort for quite a few things, so I recommend checking it out.
Now, the important thing to remember about this use of Extra Effort is that the total point value of the Power Stunt cannot exceed the point value of the base power you are using for the Stunt.
EXAMPLE: Your Blast power has a cost of 2 PP/rank. So if it is a 12 rank Blast, it has a value of 24 PP. If you decide to Power Stunt to add the Area Effect extra, the base cost increases to 3. Because you have 24 PP available for this Power Stunt, your Area Effect Blast will be an 8 rank effect.
The rules state that "The temporary power feat lasts for the duration of the encounter or until you choose to stop maintaining it, whichever comes first."
To clarify, powers that CAN BE MAINTAINED last for the duration of the encounter or until you stop maintaining them. Blast generally can't be maintained unless you buy it to a (potentially very messy) non-Instant duration. So it, and other Instant duration powers, are basically one-use. Try not to miss.