Blast Finishers?
A blast finisher is a skill that can create a combo effect by interacting with a combo field. All blast combos create area effects.
http://wiki.guildwars2.com/wiki/Blast_finisher
It seems you’re an engineer so you can lay down fire field on your flamethrower and then you can stack might by using blast finishers
http://wiki.guildwars2.com/wiki/Combo
For a combo, you need a field, and then you need to activate a finisher in (or through) that field.
The blast finisher associated with your healing turret is on the skill to detonate it. . Which will be F1 by default, after you’ve put down the turret.
For engi, you invoke the turret blasts by detonating them (pressing the appropriate F1, F2, F3, F4 after the turret is deployed).
So for healing turret:
1) press 6 to deploy
2) do not press 6 again to overcharge the turret (This will put out a water field, which can interfere with what you’re trying to blast).
3) Press F1 to detonate. This will trigger the blast.
Rifle turret is similar to healing turret, just different buttons.
Thumper turret is your best blaster. To fully utilize it:
1) Deploy the turret.
2) Overcharge the turret.
3) Detonate (you’ll get 2 blasts here if you overcharge, one otherwise)
4) Use the tool belt skill for a third blast.
I’m still somewhat noob at engi, if anyone spots an error on my part, feel free to correct it