For the piecing or for decorative stitching?
I have one for my Bernina and I've used it for decorative stitching (I don't embroider), either on a quilt top or on the quilt sandwich. It's a really cool attachment.
http://www.berninausa.com/product_de...-i172-sUS.html
I don't see how it would work for piecing.
eta:
Here's the Babylock equivalent.
http://www.babylock.com/accessories/?Product_ID=BL-CSA
Looks like it works exactly like the Bernina.
eta -again:
Ah, okay, I get it now.
It's not really piecing - here's the Fons & Porter rendition:
http://www.babylock.com/ftp/projects...th_a_twist.pdf
A sort of sew DP. Nifty idea.