Conditional line break
A conditional line-break will be activated only when necessary, that is when the conditional line-break is encountered within the right-fill-area as which is typically the remaining 30% of the line assuming that the line-fill-percentage is set to 70%.
Using conditional line-breaks instead of fixed ones makes it easy to change scaling and other layout parameters without the need to adjust explicit line-breaks.
Use the button 
from the bar line menu or the corresponding shortcut-key <shift-i> to insert a conditional line-break.