Both of these work together to form a simple machine. You can't have one without the other.
If you try to turn just the axle itself, then you'll find it takes a lot of work. This is because the inertia of the axle wants to keep the object at rest. Also, you won't have a lot of torque due to the small radius compared to what a doorknob can offer.
Using a doorknob is like putting a (steering) wheel on an axle. This increases the radius and therefore increases the torque. You put in less work into the system and get more out of it.
The reaction begins with a hydride nucleophile reacting with the ester carbonyl carbon to form the tetrahedral intermediate.
The carbonyl reforms to produce an aldehyde with the loss of the alkoxide ion.
The resulting aldehyde undergoes a subsequent reaction with a hydride nucleophile to form another tetrahedral intermediate. The carbonyl is not able to reform, because there are no stable leaving groups.
Therefore, the tetrahedral intermediate is protonated to produce a primary alcohol.
