Why should Bak work prior to the intermediate sprint?
To be honest, HTC's best case scenario is for the break to fail PRIOR to the sprint, then have Cav take max points there. If others want to chase it down, Bak SHOULD sit and let it come together again.
I'd think he should only start working after they are past the sprint (and he should probably attack just before to try to take points... even though the sprinters in the group really aren't in contention for green. Hushovd is sort of close.
Honestly though... they aren't coming back prior to the sprint so it's probably stupid not to work.