After playing through a full game of Chaos Mode, you end up with a slot machine.
The slot machine has 5 slots; Each slot is unlocked depending on the score you receive at the end of the full game.
Eg, first one is free, second 20k, third 200k, fourth 1million, fifth 45million scores.
(these were example numbers - the numbers will vary depending on which map you are playing as well.)
Once it calculated how many slots you have earnt, it spins them all.
There is a small chance of gaining an amp upgrade, most of the time it is dollars or overdrive.
This is very much a luck based achievement but the better you/your group does in the game the more spins your receive.
I have now got this achievement, and can confirm I received it from upgrade in 3 spin slot. I received Bounce-Splosion Lvl 2) - which is an Epic Amp)
Then got another amp 2 games later in 4th slot.(Bear Force One Lvl 2).
Can also confirm second slot gave amp.
Still have not seen free amp in first slot.
Fiatim confirmed that the prizes from slots are already calculated at the end of the match, so the achievement will pop just before you get to the slots screen.