The Multi-hack is used to increase the number of times a Player can Hack a Portal before it goes to the Burnout state. The Multi-hack is one of the 5 new Portal Mods from Ingress v1.28.1. In combination with Heat Sinks its a killer mod for big Portal farming!

Use:

Each deployed Multi-hack will increase the number of times you can hack a Portal depending on its type. Here are the types:

Common = +4 times
Rare = +8 times
Very Rare = +12 times

But only one of the rarest MH will add full number of hacks, all others will give 50% of their stats. For example 1 R MH and 2 VR MH gives 8/2+12+12/2 = 22 more hacks.

So with the initial 4 available hacks, you can get up to 4+12+(12/2)*3 = 34 hacks before sending a portal to the burnout state, and to hack all of them you will need 170 minutes, or almost 3 hours.