Cooldown timer – Instant reset
When a Heat Sink (regardless of rarity) is installed to a portal, the current cooldown timer is immediately finished. What does this mean? Say I hacked a portal and have 300 seconds to wait until I can hack again. When I install the Heat Sink, that counter is set to 0, allowing me to hack the portal instantly.
This instant reset is only given to the Agent that deployed the mod. Even though my timer is reset to 0, you still must wait the the cooldown period before hacking again. You do, however, still benefit from the +% cooldown decrease given from the Heat Sink.
Burnout Reset
Typically you only get 4 hacks on a portal before burning it out. Multihacks of course add onto the burnout counter, allowing any Agent to hack beyond it’s burnout limit. Heat Sinks (regardless of rarity) now reset the burnout period to 0 for the Agent that installed the mod.
This reset is only a one time reset of the burnout counter for the Agent that installed the mod.
Example 1:
I’ve hacked a portal 4 times and burned it out. Adding a Heat Sink resets that burnout counter to 0, allowing me to hack 4 more times, with a total of 8 hacks. Other Agents still only get their original 4 hacks with no burnout reset.
Example 2:
I’ve installed a Common Multi Hack and hacked a portal 8 times before burning out the portal. Adding a Heat Sink will reset that counter to 0, allowing me to hack 8 more times.
Stacking
Even though only the Agent installing the mod gets the Cooldown and Burnout bonuses, these bonuses can be obtained again.
Example:
I stumble a portal with a VR Multi Hack (deployed by another Agent) giving me 16 hacks. I install a Heat Sink after burning it out, giving me another 16 hacks. After that, I install another Heat Sink, giving me 16 more hacks, 48 in total.
http://decodeingress.me/2014/03/21/next-generation-heat-sinks/