Rolling vs Crossing the hands in form openings

[Marnetmar]

Anyone know why some of Yip Man's students roll their hands over each other akin to mainland branches and others (including Yip Man himself from what I can tell) simply cross their hands down and then up to "define the centerline"? It doesn't appear that Yip Man himself ever rolled his hands, even in his early Foshan teachings, if we go by pupils like Kwok Fu.

Branches that roll:

Leung Sheung (Except for Kenneth Chung's people from the '80s onward I believe)
Ho Kam Ming
Leung Ting (learned partially from Leung Sheung)
Chow Tze Chuen
Ip Ching for Chum Kiu and Biu Jee but not Siu Nim Tau

Branches that don't:

Just about everyone else from what I can tell

[LFJ]

Those who roll, from what I've seen, have all sorts of application ideas rolled up in those actions (taan-sau, gaang-sau, kwan-sau, back).

My line (WSL) doesn't roll and in short it's more of an abstract illustration of man/wu concepts, done low to train low elbows in beginners.

[Sihing73]

Only speaking for myself;

I find that the idea of rolling allows me to not only define the center line but also start applying energy in an upward and outward manner.

I used to do a drill where my arms would be perpendicular, one on top like in Double Lan Sau in the form.
I would then roll both arms, think speed bag work, and gradually extend to throwing punches.
Sometimes going back to the top over position and throwing punches sporadically from there too.

It was a nice drill to coordinate movement of both hands.

I was taught to roll the arms in the forms and there were some applications alluded to, whether or not those were effective is another matter.
Having said that, I have always said each line has someone who can make their approach work so there is benefit in the various approaches.