A Few Words About the Concept
To most of us here it’s nothing new as Swami Ji himself has numerous times explained in various discourses that on the path of devotion it is very important to stick to a single form of God. It helps one establish a bond with the Deity thus helping speed up the progress.
Precisely it means devotion which is directed towards a single form only (not going after other forms thinking those might be better or quicker to respond).
The word also appears in Srimad Bhagavada Gita (13.8-12).
Mother Divine’s Devotion for Lord Shiva
The incident is from Sri Ramacharitamanas of Goswami Tulasidas. When Mother Parvati was engaged in harsh tapas (penance) for pleasing Lord Shiva, Lord Shiva sent the great seven Seers (Saptarishis) to test her love.
They went with a plan to try to divert her from path to Shiva or persuade her to give up this pursuit and look for better options. Accordingly, they go and start saying bad things about Shiva and praising Vishnu:
तेहि कें बचन मानि बिस्वासा। तुम्ह चाहहु पति सहज उदासा॥
निर्गुन निलज कुबेष कपाली। अकुल अगेह दिगंबर ब्याली॥
Trusting his (Narada) words, you want such a husband who is indifferent by nature, devoid of qualities, shameless, badly dressed, who wears garland of human skulls, having no great family lineage or home, doesn't wear clothes and keeps body adorned with snakes.
These are followed by few more words depicting Shiva as bad.
Then praising Vishnu:
अजहूँ मानहु कहा हमारा। हम तुम्ह कहुँ बरु नीक बिचारा॥
अति सुंदर सुचि सुखद सुसीला। गावहिं बेद जासु जस लीला॥
दूषन रहित सकल गुन रासी। श्रीपति पुर बैकुंठ निवासी॥
अस बरु तुम्हहि मिलाउब आनी।
You can still listen and do as we say, we have thought of a very good groom for you. He is very handsome, pure, soothing, and pleasing; whose glory and pastimes are sung by the Vedas. He's free of imperfections, consisting of all the good qualities, Lord of wealth (of Goddess of Wealth) and lives in Vaikuntha. We shall bring such a groom for you.
सुनत बिहसि कह बचन भवानी ।
महादेव अवगुन भवन बिष्नु सकल गुन धाम।
जेहि कर मनु रम जाहि सन तेहि तेही सन काम॥
जौं तुम्ह मिलतेहु प्रथम मुनीसा। सुनतिउँ सिख तुम्हारि धरि सीसा॥
अब मैं जन्मु संभु हित हारा। को गुन दूषन करै बिचारा॥
Listening to them, Bhavani (Maa) laughed and said.
I agree that Shiva is embodiment of all bad qualities and Vishnu is abode of all good qualities, but in whom one's mind has absorbed and engaged, one's only business is with him.
O Sages! If you had met me earlier, I would have heard your suggestions with all reverence, but now I have given my life to Shiva! Now who cares about good-bad qualities.
And now, Mother says the most beautiful lines full of devotion and surrender which capture the essence of avyabhicharini bhakti most accurately:
जन्म कोटि लगि रगर हमारी। बरउँ संभु न त रहउँ कुआरी॥
तजउँ न नारद कर उपदेसू। आपु कहहिं सत बार महेसू॥
I'll stay persistent for crores of lives - Either I'll marry Shambhu (Shiva) or I'll not marry at all. I'll not give up teaching of Narada even if Lord Shiva himself asks me seven times to do so.
This captures not only the fact that you must stay devoted and focussed on a single path and one form of Lord but also not giving up following your Guru’s instructions even if the greatest of sages or even your Lord himself asks you to (it ought to be a test mostly). Here, Narada gave Mother the suggestion to please Shiva and Mother at one place regards him as Guru for that in Ramcharitmanas:
गुर कें बचन प्रतीति न जेही। सपनेहुँ सुगम न सुख सिधि तेही॥
One who doesn't have belief in his Guru's words, can't get happiness or any Sidhhis even in dreams.
Jai Sri Hari
Sri Ramcharitmanas – bharatdiscovery.org