Those were marked as "secret" by DaB, along with a few others. To find them, rename a copy of WorldOfGoo.exe to WorldOfGoo.txt and read it.

Thrust I think means it launches when it's on fire? Still testing it.

I used both of it with the "goomatz_Thruster" ball in "keep on running"

decay is easy. It's the live time(in seconds) of a ball. The time is running from levelbeginning or the moment a ball get spawned by a dispender.

thrust adds a force to an attached ball. It only seem to work with two strands. A negative value will make it pull in direction of the bisecting line of its two strands. A positve value will make it push into the other direction.

@Baft as Brush,
I hope didn't spoil anything with this comment. If so, delete or edit it.

I din't know any of that!!! Decay seems interesting but I don't get the thrust.

I found thrust in goomstzs level. Experiments told me, that it works realy strange with one-strand balls.
I used decay parameter in my level The Demo Tower (tm).

With thrust of +1, does it use the bisector of the minor arc or the major arc? What if you make it exactly 180 degrees in the editor?

Updated : http://goofans.com/developers/game-file-formats/balls-xml
Hope that helps!

@Goomatz
Not really "secret", but I did uncover / discover them and figure out how they work, and thought I might find a cunning use for them...

@AP
thrust is the amount of force, not direction.
Direction is pretty complicated, hope the new explanation helps... a bit.

Are the "thruster" particles automatically used for the thrust? Do they always follow the thrust direction? What if you do something like this:

O - O - O

where the middle one is a thruster and the other two have the same Y value as it?

Lots of questions.

During my tests balls were giving thrust only if they get attached by the player with two strands. In any other case (attached from levelbeginning, strand uneven two etc.) I couldn't notice any force affecting the structure.
In case one of the two strands of the thrusting ball breaks the force doesn't stop, but it doesn't start if the strands get reduced from three to two by death or detached strand partner.

I hope my bad english isn't too bad for the understandability.

@ AP
The "thruster" particle always follow the top of the ball, which is the direction of thrusting. But if the ball rotates after it's attached the "thruster" particle will point in a different direction than the force is pushing/pulling to.

@ DaB these are just the observations I made. Please inform me if I'm wrong, but If I'm right can you correct that on the balls.xml page?

So the particles are automatic? Any "thrust" ball has those particles?

(sorry, writing this from a computer without WooBLE)

@ AP
I just added particles with:
effect: "thrusterish" (a custom one but very similar to the "thruster" effect)
state: "attached"
overball: "false"

And as far as I know there hasn't been any other "thrust" balls than "goomatz_Thruster"

What about the original levels? What were these used for?

Neither decay nor thrust are used by any original balls.
What also mean they aren't used in any original levels.