Class

Mech

Heart of the Machine · levels at 3 / 6 different pieces

Every Mech piece: Mechs receive +50% healing from all sources.

Rewards are given at the start of the next preparation phase. The in-game description calls these pieces “Inventors”.

3
Win a round with at least one Mech still alive to get a Heart of the Machine (a free shop refresh).
6
The reward has a chance to be a Golden Heart (pick from 5 random Legendary pieces): 20% per courier level above 5 (level 6 = 20%, level 10+ = 100%). Otherwise you get a normal Heart.
Note: The tooltip says level 6 always gives the Golden Heart. It is a chance: 20% for each courier level above 5, so 0% at level 5 and 100% at level 10.

Official in-game tooltip

Inventors arm themselves with the latest technology that allows them to receive +50% healing from other sources.

Class Perk: Heart of the Machine
Active when you have at least (N) different Inventor pieces on board. (Multiple effects stack)

(3) Inventors: Summon an AWESOME Invention after winning a home battle last round with at least 1 friendly Inventor who survived.
(6) Inventors: Heart of the Machine now summons a Golden AWESOME Invention.

Inventors are usually the ones within a race who possess the most profound knowledge, and the are ambitious in discover new methods and creations. Though most of the time the inventions are problematic and not-so-reliable, they do invoke miracles sometimes.