Ramana Maharshi talks of “Hridaya Granthi” - The Knot in the Heart. When this knot is pierced and dissolved, that starts the first stage of Non-duality, where the non-experience-experience is flavored by Unconditional Love (which later also gets transcended as one refines the non-dual “knowing”).
Above and below every Chakra, there are two knots, energetic in nature, which are more like psychic apertures that restrict the smooth ascent of the Kundalini-energy - you can literally feel the friction once you experience such an awakening. The ones related to the heart are emotional ones related to love, kindness, and compassion.
For each of the different knots and chakra pair, there are different methods to ensure smooth energy flow - Jnana (knowledge/self-inquiry), Bhakti (love and devotion), Karma (service), Kriya (activity like breathing, meditation), etc. For the Heart, it is Bhakti - love, devotion, and compassion. To dissolve the constraints of the heart, Bhakti Yoga or the Yoga of Love and Devotion works very well. This love can be an overwhelming sense of love and devotion towards a higher vibrational entity such as deity/guru/god/gods; or toward other fellow human beings or animals or even a selfless cause.
Loving Kindness Meditation is a great tool to dissolve the knot of the heart as it triggers certain alchemical pathways that are otherwise hard to activate.