SpinUp is the time (and effort) it takes for a developer(etc) to become productive when they move into a new area either outside of their expertise or into new techniques outside of their expertise.  Until they are "spun up", there's not much they can contribute to the new task -- it offsets their productive time.  This can also be a debt against a seasoned developer's time as they try to shepherd the person by showing/explaining/teaching/etc. -- anything that takes away from the seasoned developer's productivity.  
The portion of the resource's time (developer) is spent just trying to come up to speed on a project -- to get to the point where they are able to actually contribute enough to offset their SpongeCycle.