Sheer Force increases the power of moves that have beneficial secondary effects by 30%, but removes those additional effects.
Beneficial effects are defined as those that are positive for the user, such as:
Causing a status condition on the opponent (e.g. Thunderbolt)
Reducing the opponent's stats (e.g. Crunch)
Increasing the user's stats (e.g. AncientPower)
Non-beneficial effects, that Sheer Force will not activate for, include:
Recoil damage (e.g. Double-Edge)
Lowering the user's stats (e.g. Superpower)
If the user is holding a Life Orb and uses an attack that activates Sheer Force, it will not receive the 10% damage from Life Orb, but it gets boosts from both Life Orb and Sheer Force (total of 69% base power increase).
Flamethrower normally has a 10% chance to burn the opponent. With Sheer Force, Flamethrower's base power is boosted from 95 to 124, but it will never cause a burn. If it is holding a Life Orb, the damage will be increased from 95 to approx. 161, without the damage dealt to the user.
There are only 9 Pokémon that have this ability, one of which is Mega Evolved.
Pokémon with this Ability