When we are unwearied in doing what He asks
When we do not fear man
When we seek His will
When we are obedient.
When we do these things, the Lord will bless us in His own way.
Nephi had gotten to the point in his life, in his character, where his choices were completely congruent with God's will. Because of that, and his unweariness in his work and obedience, God gave him the ultimate blessing: to have complete access to use God's power at will.
I can't even imagine. What an amazing man he was!