Figuring out what makes French mare Jiayuguan tick hasn't been easy.

But trainer John Thompson reckons he has solved the puzzle and her form attests to his theory.

"She took a while to settle in but we've worked her out now and she's going great guns," Thompson said.

"Basically getting her to 2400 (metres) was the first key. She is a genuine, dead-set stayer.

"The second key was you don't need to work her too hard, she's very clean winded. She's a typical French horse with a very light sort of frame and does very well on not much work."

Jiayuguan has won her past two starts in narrow but emphatic fashion, both times producing a finish from back in the field.

She will chase her first black-type win in the Christmas Cup on Saturday, run over her pet distance of 2400m.

Having begun her current campaign in August, Jiayuguan is likely to have a break regardless of the result at Randwick, although her performance will determine what races she is aimed at.

"Mares in form, they seem to hold their form and she's given no indication she's gone off the boil," Thompson said.

"She looks fantastic and her work has been strong."

A $3.20 Christmas Cup favourite, Jiayuguan is a $26 chance in first Sydney Cup markets released this week.