The tooltip initialy describes the relative constraint between 'Bricklaying' and 'Roofing', saying that

• 'Bricklaying' most finish no sooner that 10 days before 'Roofing' starts.
• 'Bricklaying' most finish no later than the day 'Roofing' starts.

When dragging the project along the plan, the relative constraints are respected, ie when 'Bricklaying' meets an obstacle it will rest until the span between it and 'Roofing' reaches 10 working days, at which point it will start moving again.

I this example the 'Fundation' activity didn't move at all. That's because it has allready started.
As a result the relative constraint between 'Fundation' and 'Bricklaying' is violated.