Two examples...
Now flick took a wrong turn on his way home, and all of a sudden he's heading past a gang house. He's had some bad history with the gang, and he can't tell they have spotted him. Now he's not going to make it away on his scooter, but he has a job tomorrow,and if he is recovering from physical he's not going to be able to make it, and he needs that payday.
Flick is a Magic 6 Mage who likes fire.
He sees the gang is starting to pull out weapons, and some are getting on their bikes. Quickly he fires off a force 12 fireball (max he can cast). This has a huge area with its radius of 12 meters, so is going to get all the bikers outside outside there pad and also damage the front of the building.
The drain for the fireball is Force-1, so in his case 11 (9 if he has a fetish for it),'and it will be stun if he gets six hits (magic) or less, but physical if he gets more than six hits.
He can't risk the physical, so he uses six reagents which mean the maximum hits he can get is now limited to six; thus stun only.
The poor bikers, they don't get to dodge as it is an area spell, so,they will be soaking 13-18 Physical Fire with an AP of -12 (force). That's going to hurt, and their pad is going to get a good burning up the front.
Odds are good Flick will take some drain, but it will only be stun.
Over night Flick recovered the stun and is now working with his team. Clearing a building they charge into a busy room, lots of people here and all their guns are looking at the team. The Sam's being the fastest starts shooting, and so do the opposition. Come Flicks phase, he wants to do as much damage to as many as possible, but he doesn't want to injure them team. He figures he can get a 4 meter radius fireball to cover all the bad guys but mis the team. Sure it may start a few fires, but they are moving through the building pretty quickly.
He casts a force 4 fireball, for the radius. Now he really wants to maximise the hurt, and now he's working he is willing to risk a little physical if need be.
He throws his edge in, so the hits h get can easily be over six, his magic score; thus putting him into physical.
Luckily it's only a force 4 fireball so even if it's going into physical he is pretty sure he can drain the (force -1) 3 boxes.
The guys in the roam, they will be hit and trying to soak 4DV (plus hits), at -4 AP
Now Flints not going to clear the room with this fireball (unless he rolls madly well), but the bad guys are in fire, his team isn't and with the low drain code he believes he can keep fire It phase after phase without two much risk; though next time rather than edge he may chose to use reagents to increase the number of its he can possibly roll above four if he believes that is going to be an issue.