The relationship between the FTSE100 and cable reflects the fact the the major constituents happen to have significant $ exposure.
BP
Shell
AZN
GSK
DGE
HSBC
Therefore, their translation of profits can be materially impacted by the $ performance.
Weak dollar can reduce their apparent profits when translated into sterling.
(depends on hedging policies).
Of course there are other impacts but you get the point
Remember it is all about perception of facts.
Take yesterday, Molins reported in line numbers
but warned about its 2004 outlook re dollar.
Share price has since tanked, down 11% yesterday
and another 8% today.