Only an empowered personality can distribute the holy name of the Lord and enjoin all fallen souls to worship Krishna. By distributing the holy name of the Lord, he cleanses the hearts of the most fallen people. Therefore he extinguishes the blazing fire of the material world. Not only that, he broadcasts the shining brightness of Krishna's effulgence throughout the world. Such an acarya, or spiritual master, should be considered nondifferent from Krishna-that is, he should be considered the incarnation of Lord Krishna's potency.
The spiritual master can instruct the disciple though many different formats.
When the teacher, the spiritual master, is praying for the mercy of the Lord to enlighten the student, then by the blessings of the Lord the student gets the blessing of knowledge.
A disciple serves the spiritual master with the sole purpose of getting instructions from him.
Guru is heavy, because he carries Lord Krishna in his heart.