Use the Physics if you Wont Use the Physic

"Give me a lever, I will lift up the world":

 this is what Archimede from Siracusa said to demonstrate that a ship could have been dragged to the beach using levers. 

The chef knife has  handle is shorter than the blade so that you can arrive totally to the surface of the cutting board.

The proper movement is: keep the knife point on the board; place the food to be sliced ( onion, chocolate, parsley...) under the blade, about at half ways of its lenght; start to press the handle down at the same time sliding the blade forward you. In this way you cut your food. Then raise the handle, still keeping the point on the board, and pull the knife back, toward you.
In this way you will you the knife as a lever. 

This is Karen, applying perfectly the physics principle of the Lever.

 Karen and Roland  are slicing onions for the Minestrone.

This is Roly.

When you use a Chef knife properly, you do not need to apply much strenght. Just you need to apply the Lever principle.

Thus, either you are as muscolar as Roly or as delicate as Karen, the result of your "slicing" will be the same: perfect and even.