Hmm ... can't say I've come across the term 'Jack Pin' before....
wonder why the word 'Jack'?
I knit with a long, straight needle tucked under my right arm, as do alot of knitters of my vintage in this area. It's technically known as 'Shetland' knitting.
If you open the link, scroll down a little bit and you'll find a diagram of this method. The orange text to the left is also a link to a history of northern English - mainly Yorkshire Dales - knitting and shows how a belt was used to stabilise the right needle.
You can still buy these belts from a firm in Scotland.
All the Best
Ellie (the Lancashire one)