Procs Per Minute
Overview
Procs Per Minute (PPM) is "average" number of times per minute an enhancement's special effect will attempt to fire. This mechanic is used exclusively by Store-Bought Enhancements and Archetype Enhancements. Invention System enhancements continue to utilize a flat percentage chance per power execution.
Procs Per Minute are calculated off the base recharge of a power, not its final enhanced or buffed number. So a power that takes 12 seconds to recharge will have the same PPM as the same power enhanced and buffed enough to halve its recharge.
Guaranteed Proc
The formula for calculating a 100% chance of fire is 60/PPM. For instance, if an enhancement has a 5PPM rate, then 60/5=12 seconds; a power with a base recharge of 12 seconds will guarantee a proc every time it executes, even if the power's final recharge is less. If an enhancement has a 1.5PPM rate, then a power needs a base recharge of 40 seconds to guarantee a proc.
Developer Comments
Arbiter Hawk explained it in several posts on the official forums regarding Store-Bought Enhancements' Procs and Archetype Enhancements' Procs:
This is a bit of a trick question - these procs that you mention function like store-bought IO procs, so their chance to proc with any given power activation fluctuates with the power's recharge time and some other factors. The long and short of it is that you should put the proc in a power you like to use, but this Proc-per-minute functionality does not advantage, say, Neutrino Bolt over X-Ray Eyes, if you like using X-Ray more. Slot the procs into attacks that you use, and you'll see them go off fairly regularly.
It is not limited to its displayed number of procs per minutes - that is representative of the average number of times it will proc over a minute if you use a single-target power every time it comes off recharge. You can still "get lucky", roll high numbers on the under-the-hood dice, and get many more than 4 procs in a minute. Its just that instead of a flat 20% chance to proc (which gives higher proc uptime per minute to a power that can be used more times per minute), the chance of proc'ing is now variable with the recharge (and other special factors) of the power that you're enhancing.
It's base recharge time. If you have a single target attack slotted with a 2 ppm IO, the power would need to have a 30 second recharge to have a 100% proc chance. You could then IO it to ~100% recharge and combine that with 100% global recharge bonus to get the power's recharge down to 10 seconds, and it would still have that 100% proc chance. This is how all of the store-bought IO set procs work, as well as the ATO procs that are not global bonuses. This changes the math on "optimal proccing" significantly from the traditional "flat rate" approach, and causes the performance delta of optimal and non-optimal to be much smaller, so you can really slot these in any power you use and you will see them be effective.