Gang sau

When you roll down Gang Sau it is the same movement as letting it drop down on its own. There is no need to think about the movement.

When you let the arm drop, It naturally pivots on the shoulder joint first.

This is the first thing to remember. The second is that, as always, you should drop the shoulder onto the elbow. 

Sifu describes this as 'putting your arm on the table.'

Imagine you are stood next to a High table and you lean on it. You're not pressing down but a large proportion of your weight is going into the tables structure. 

Once your shoulders are dropped then you can rotate the shoulder joint and expand the elbow joint.

Easier said than done!