The Sun 73 is a modern form designed for competition. As such, it is between 6:00 and 6:30 in length and eliminates the repetition in the traditional long form. Also, as is usually the case with modern forms, it is more left-right balanced than the old form.
My current teacher here in Austin, Grandmaster Aiping Cheng, learned from Sun Lutang’s daughter, Sun Jian Yun. Years ago, the Chinese government made a video of Aiping performing the Sun competition form, and this is the standard for judging the form. Unfortunately, the copy on YouTube is incomplete (only the first three minutes) and not the best video quality.
The name of the form is 孙式七十三式太极拳竞赛套路, Sūn shì Qī Shí Sān Shì Tàijíquán Jìngsài Tàolù, which translates as Sun-style 73-Step Tai Chi Competition Form. Below, Gao Jiamin demonstrates the Sun 73 with Li Deyin providing the names of the movements.
I have compiled a list of names by transcribing from this video. PDF: names of movements in the Sun 73.
Li Deyin has also made a teaching video in two parts, each part about an hour long.
Part 1: Beginning to Separate Right Foot (sections one and two)
Part 2: Step forward and punch down to end (sections three through six)
The 98 and the 73 start out the same—the whole first section is the same, in fact. But then in the second section of the 73, several of the movements occur on the unfamiliar side: Lanzhayi is on the left, single whip is on the right, and cloud hands travels to the right.
In section three, ban lan chui is on the opposite side from usual. The four corners of Fair Lady Works the Shuttle (section five) are in a different order in the 73; the order in the 98 is the same as in the Yang 108: SW/SE/NE/NW. In the Sun 73 the corners are SW/NE/NW/SE.
Finally, here is Gao Jiamin in back view:
As usual, I got Jesse Tsao’s instructional video from Taichihealthways.com. Between that and Li’s videos above, I learned the form well enough to schedule some private lessons (outside, masked) with Aiping. I actually learned the 73 first, and found it pretty easy to move up to the 98 from there.