This is a very common mistake I see among Xingyi Quan practitioners.
It is so common, I start to doubt if I was taught correctly by my Shifu.
So I looked up old books and manuscripts just to double check.
Nope, I’m pretty confident I was taught correctly.
At least what I learned aligns with all the Xingyi texts out there.
Go check them out if you have doubts.
The most common mistake I see with with Xingyi Quan Drilling Fist is their fist travels along a straight path, diagonally. I added the red arrow to illustrate the path of the hands.
This is WRONG. Some styles punch like this, and it is okay. The shortest distance between two places is a straight path. Yes, I get it. No problem with that punch at all, but it is not Xingyi.
In Xingyi, the path of the fist is specifically called out. Our hands travel in a circular (more like oval) path, while we twist our forearm like a drill (hence drilling fist).
Here’s the picture straight from the book, fist going out:
Here is retracting the fist, coming back:
For those of you that want to do your own research, the book I referred to is:
形意拳入門講座 // Introductory Lectures to Xing Yi Quan