I think you are talking about either
1. Diagonal fly - Your shoulder is under your opponent's leading arm, or
2. Wide horse ruffles mane - Your shoulder is above your opponent's leading arm.
In order to make "shoulder strike" work, you will need to follow 2 major principles.
1. Forward momentum - In order to generate the forward momentum, you have to step in your back leg and then step in your front leg again.
http://www.youtube.com/watch?v=T7qVSX4M_Sw
2. Leg control - In order to prevent your opponent from getting away, you have to use your front leg to hook your opponent's back leg.
http://www.youtube.com/watch?v=VQHiy...ature=youtu.be
Because both principle are missing in Taiji training, the Taiji "scissor uprooting" doesn't work very well. Taiji guys don't like to use momentum. They don't like to commit. Taiji guys also don't like to use leg techniques. They don't like to stand on one leg. This is why Taiji can never be an effective throwing arts system.