Procs in Toggles

How does a Proc socketed into a Toggle work? From what I've read on the Wiki, the Proc buff will be in effect for 100% of the time that the Toggle is on. If this is true, wouldn't socketing Gaussian's Synchronized Fire-Control: Chance for Build-Up in Tactics give you a 100% uptime 100% damage boost?

Procs slotted into toggles (or auto-powers for that matter) will check every 10 seconds with a chance to go off, and when they do, they last for 120 seconds, making them effectively perma.

Guassian's Chance for Buildup is an exception though, only lasting for a few seconds at a time and only has a 5% chance to proc (i think 5%?), making it proc off much less often than say a Numina's +/+ or Miracle +Recovery.

That explains why you don't see everyone doing it. It is a 5% chance, and it last 5 and change seconds. If it only checks every 10 seconds then it's too random to be worth 2 powers, a power pool, and an expensive Unique.

Gaussian proc is pretty much only useful if you're going for the 6 piece def bonus.

I agree -- the gaus proc is only worth it if you're 6-slotting. Not worthless, but not amazing either.

Note that the Proc120s have been 'fixed' for Issue 18. They were never intended to last for 120 after a Toggle is shut off. They now immediately shut off when the Toggle is shut off.

While the Toggle is on, they do continue to work the entire time the Toggle is on.

In Passive powers, they are always on while the Passive is working.

In Click Powers, they will work for 120 seconds (pulsing or checking every 10 seconds for the 120 second duration).

Zombie is talking about the Proc120's, like Stealth, +Recovery, etc.
The 'chance for damage', 'chance for build-up', etc are not 120's.

Next question: Do Build-Up effects stack? I was thinking of 6-slotting Guassian's into Build-Up on my Electric Melee / Willpower scrapper, 'cause it'd be pretty cool to occasionally have a +200% damage Build-Up

There are two types of procs in the game.

Normal procs (Gaussian's "chance for buildup", assorted "chance for damage", etc.) are checked every time the power they're slotted into is activated, or if the proc is slotted into a toggle or auto power, the proc is checked every 10 seconds. If the proc goes off, it does exactly what it says: applies damage, or gives you ten seconds of Build Up, or whatever.

Proc120s (the Numina's unique, stealth IOs, etc.) always have a 100% chance of going off. If they're in a click power, they give you 120 seconds of their bonus every time you activate that power (but they won't stack with themselves). If they're in an auto power (say, Numina's in Health), they will be activated every 10 seconds, again with the "no stacking" rule, making the effect permanent. If they're in a toggle (say, Unbounded Leap +Stealth in Sprint), they'll be activated when the toggle is, checked every 10 seconds (no stacking, as always), and deactivated when the toggle is turned off.

The behavior of Proc120s in toggles is what is changing in I18: prior to I18, they would activate when the toggle did, be renewed every 10 seconds while the toggle was on, and wear off 120 seconds after the toggle was turned off.