Heart Jewel contains two essential practices of Kadampa Buddhism that were revealed by the Wisdom Buddha Manjushri.
The first is a special Guru yoga focusing on Je Tsongkhapa through which we can purify negativity, accumulate merit and receive blessings. By relying upon this practice we will naturally accomplish all the realisations of the path of Sutra and Tantra, and in particular attain a very special Dharma Wisdom.
The second practice is a method for relying on the Dharma Protector Dorje Shugden. Through this we can overcome obstacles to our practice and create favourable conditions so that we can nurture and increase our Dharma realizations.
By engaging in these practices regularly and sincerely we will reap a rich harvest of pure Dharma realizations, and eventually come to experience the supreme joy of full enlightenment.
For a more extensive explanation of these practices, see the book, Heart Jewel.