Tell your mate he's a wanker.
As with any petrol ignition engine, you can't adjust / set the timing while the engine is off. It needs to be running - at normal operating temp - and you check the timing with a timing light while you make the adjustments.
If you retard the timing, part of the burn, and therefore the power production, happens too late, and can even happen after TDC - as the piston is already on its way down the bore. Doesn't make for good power production. Advance it and you ensure the maximum burn rate, and therefore maximum power production, occurs just before TDC. Advance it too far and you induce pre-ignition (pinging / detonation)