Offering to the Spiritual Guide is a special Guru yoga of Je Tsongkhapa in conjunction with Highest Yoga Tantra. It was compiled by the first Panchen Lama, Losang Chokyi Gyaltsen, as a preliminary practice for Vajrayana Mahamudra. The main practice is relying upon the Spiritual Guide, but it also includes all the essential practices of the stages of the path (Lamrim) and training the mind (Lojong), as well as both the generation stage and completion stage of Highest Yoga Tantra. All these practices were given by Buddha Vajradhara to Manjushri who in turn passed them to Je Tsongkhapa in the Kadam Emanation Scripture.
The essence of Guru yoga is to develop a strong conviction that our Spiritual Guide is a Buddha, to make prostrations, offerings, and sincere requests to him or her, and then to receive his or her profound blessings. According to the Guru Yoga of Offering to the Spiritual Guide, we develop conviction that our Spiritual Guide is the same nature as Je Tsongkhapa, who is an emanation of the Wisdom Buddha Manjushri.
By relying upon Je Tsongkhapa, our compassion, wisdom, and spiritual power naturally increase. In particular, because Je Tsongkhapa is an emanation of the Wisdom Buddha Manjushri, his faithful followers never experience difficulty in increasing their wisdom. There are many other benefits from practising Offering to the Spiritual Guide. These are explained in the book Great Treasury of Merit by Geshe Kelsang Gyatso, which contains a complete commentary to the practice.