The quality with which the forms are practiced has a major influence on the ability to apply the WT principles in actual combat.
The sequence of movements in the relatively short WingTsun forms is quickly learned, but even masters constantly refine their execution.
Naturally one could discuss countless aspects to which particular attention must be paid when practicing the forms, but the object of this series is not an exhaustive analysis of all the movements in a form. Instead Dai-Sifu Schembri provides examples of typical mistakes which tend to reoccur.

Siu-Nim-Tau

1st set:
Once the IRAS has been adopted and the fists have been pulled back as Sao-Chong, both arms are pushed forward to form a crossed Tan-Sao in front of the chest. From there they move downwards together to form a crossed Gaan-Sao, then the Kwan-Sao movement returns them to the crossed Tan-Sao position followed by a final Sao-Chong movement.

NOTE: The wrists must remain straight during the entire sequence of movements from Tan  to Gaan-Sao and back.

2nd set:
Starting with the left, this begins with a straight punch along the centreline. After fully extending the arm, the hand performs a Huen-Sao movement and the arm is pulled back with Sao-Chong. The same movements are repeated with the right.

NOTE: The elbow must be kept as low as possible throughout the punch. Even though it must of course be raised as the arm is stretched, it must always follow the lowest possible path (otherwise the result is best described as an incorrect snap-punch with the lower arm).