"The objective of the piercing palm in the Yin system I study is to apply mostly a straight forward thrusting piercing palm to attack straight to the two fatal points of the enemies body, the eyes and throat! Striking these to points even lightly can cause a traumatic effect on your enemies body as well •••• down their body nervous system and also kill the will and mindset of an enemy to fight back."
http://www.blacktaoist.com/piercing%20palm%20of%20yin%20style.html

Do you agree with this Tim and if so how do you spar in shenwu and use piercing palm against resisting opponents?

"Do you agree with this Tim and if so how do you spar in shenwu and use piercing palm against resisting opponents"

I don't practice the Yin style. You should be asking the Black Taoist your question.

Which style is superior to fighting Tim? From what you saw of both styles?

I don't think any style of Baguazhang is superior, it depends on the individual and how he trains.

The piercing palm is also used to bridge an attack by diverting and dissapating the incoming energy with rotational power. As an extension of the bridge it is then employed offensively to attack vital points as quoted by Maciej. I'm no expert but that's my understanding and how I practice.

I personally am a fan of internal styles (Ba Gua in particular) for combat effectiveness but that's just my preference. I agree with Tim that it is the indivdual, not the style, that makes a good fighter. Tim certainly didn't need help with the answer, just throwing my 2 cents in.