The concept is this simple. Your spine is inclined. In relation to the ground, the arms must lift in the backswing and drop in the downswing.
So using the spine inclination as “level,” you want your arms to remain relatively level in backswing and downswing (small variations part of the deal).
In the backswing, things like “low and slow” and inside backswings cause “arm drop.”
In the downswing, things like steep and over the top cause “arm lift.”
Both in relation to spine inclination.
People who try and “drop the club in the slot” often create too much arm drop…especially if there is a backswing issue.
It’s easier said than done, but the object is to keep the arms “level” through the whole swing.