Unification of mind

You can now investigate any object with however broad or narrow a focus you choose. But you have to stay vigilant and make a continuous effort to keep subtle distractions and subtle dullness at bay.

Effortlessly sustained exclusive attention and powerful mindfulness.
Distractions and dullness will return if you stop exerting effort. You must keep sustaining effort until exclusive attention and mindfulness become automatic, then effort will no longer be necessary. Boredom, restlessness, and doubt tend to arise during this time. Also, bizarre sensations and involuntary body movements can distract you from your practice. Knowing when to drop all effort is the next obstacle. But making effort has become a habit, so it’s hard to stop.
Practicing patiently and diligently will bring you to the threshold of effortlessness. It will get you past all the boredom and doubt, as well as the bizarre sensations and movements. Purposely relaxing your effort from time to time will let you know when effort and vigilance are no longer necessary. Then you can work on letting go of the need to be in control. Various Insight and jhāna practices add variety at this Stage.
You can drop all effort, and the mind still maintains an unprecedented degree of stability and clarity.