Simply stated, the nature of mind of the Guru, and the nature of mind of yourself, are exactly the same. There is no difference whatsoever. In Longchen Nyingthik Guru Yoga, one dissolves into the guru, and in many other guru yogas, the guru dissolves into oneself...but either way, it's the same. Visualization practices are skillful methods, but should not be mistaken for the ultimate or final "purpose"-i.e., the main point is not to develop one's imagination so that the visualization becomes vivid and "seemingly real," amazingly detailed, etc. (though these things are signs of stability and, in some sense, "success" in practice).
One uses the visualization techniques, including the "dissolving," (and in fact, all techniques, really) to effect a state of mind that allows innate wisdom to "manifest" or "appear." Of course, it's always been there, but the techniques are skillful means to allow realization of the truth to dawn in the practitioner's mindstream.
All visualization is "meditation." But not all "meditation" is visualization. The best "meditation" is nonmeditation. Remaining at rest in the nature of mind of the guru, primordially inseperable with one's own nature, is the crucial point.
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