Seriously? That's what they're calling "jum sao"? His arm goes down almost straight. Anyhow...
Everyone is familiar with the WSL version, right? It has two clear sections of taan/jam/taan and taan/gaang/taan.
The jam-sau is just a subtle action contracting the elbow. This doesn't appear in either the CWS or YKS lineages' SNT form. What they do appears to be the equivalent of the gaang-sau section, whatever they call it.
This jam-sau is what was referred to when Yip Man said he got it from Leung Bik. It clearly didn't come from CWS or YKS.