My understanding is that the move used to be called only "slice" then
some callers started explaining what is what by telling people that they
should yearn to dance with the people on their left (normally) diagonal
- that was just to emphasise what a slice is all about.
The term became popular and some people started using the word "yearn"
for the move. So slice and yearn are identical.
They are both single progressions, you yearn (or slice) towards someone
on the diagonal and then fall back opposite them. If you want a double
progression then it is a double slice - I have never heard that called a
double yearn - it wouldn't make sense as yearning is all about moving
towards someone that you want to dance with.
Double slices are usually in long lines.
In a single yearn people often work as a couple so that they have a free
hand to press up against their new opposite's hand before falling back.
Again, it emphasises who your new neighbours are.
But there is no rule as to whether a yearn/slice is as a couple or in a
long line.
And undoubtedly there are other interpretations somewhere and somewhen
in the dance world :-)
Happy dancing,
John
John Sweeney, Dancer, England john(a)modernjive.com 01233 625 362
http://www.contrafusion.co.uk for Dancing in Kent