There are two kinds of Gurus.

These are embodied in the two Murthis.

One is Lord Krishna who explains everything logically by lengthy discussions.

The other One is quiet, conveys the message by Silence,

Krishna declares in the Bhagavad Gita,

“of Languages I am Silence’

The Brahman is the embodiment of silence and tranquility.

The cessation of the modification of Chitta, is Yoga.

The turbulence of the mind when stilled is Yoga.

There is a Upanishad, Dakshinamurthy Upanishad, which reveals the Truth about Dakshinamurthy Mantra.

Dakshinamurthy Upanishad.

yanmaunavyaakhyayaa maunipaTala.n kshaNamaatrataH .
mahaamaunapada.n yaati sa hi me paramaa gatiH ..
AUM saha naavavatu . saha nau bhunaktu . saha viirya.n karavaavahai .
tejasvinaavadhiitamastu maa vidvishhaavahai ..
AUM shaantiH shaantiH shaantiH ..
AUM brahmaavarte mahaabhaaNDiiravaTamuule mahaasatraaya sametaa
maharshhayaH shaunakaadayaste ha samitpaaNayastattvajij~naasavo
maarkaNDeya.n chira~jiivinamupasametya paprachchhuH kena tva.n 
chira.n jiivasi kena vaanandamanubhavasiiti . paramarahasyashiva\-
tattvaj~naaneneti sa hovaacha . ki.n tatparamarahasyashivatattvaj~naanam.h .
tatra ko devaH . ke mantraaH . ko japaH . kaa mudraa . kaa nishhThaa . 
ki.n tajj~naanasaadhanam.h . kaH parikaraH . ko baliH . kaH kaalaH . 
ki.n tatsthaanamiti . sa hovaacha . yena dakshiNaamukhaH shivo.aparokshiikR^ito
bhavati tatparamarahasyashivatattvaj~naanam.h . yaH sarvoparame kaale 
sarvaanaatmanyupasa.nhR^itya svaatmaanandasukhe modate prakaashate
vaa sa devaH . atraite mantrarahasyashlokaa bhavanti . medhaa
dakshiNaamuurtimantrasya brahmaa R^ishhiH . gaayatrii chhandaH . 
devataa dakshiNaasyaH . mantreNaa~NganyaasaH . AUM aadau nama uchchaarya
tato bhagavate padam.h . dakshiNeti padaM pashchaanmuurtaye padamuddharet.h .. 1.. 

asmachchhabda.n chaturthyantaM medhaaM praj~naaM pada.n vadet.h . 
samuchchaarya tato vaayubiija.n chchha.n cha tataH paThet.h .
agnijaayaa.n tatastveshha chaturvi.nshaaksharo manuH .. 2..

dhyaanam.h .. 
sphaTikarajatavarNaM mauktikiimakshamaalaa\-
     mamR^itakalashavidyaa.n j~naanamudraa.n karaagre .
dadhatamuragakakshya.n chandrachuuDa.n trinetraM
     vidhR^itavividhabhuushha.n dakshiNaamuurtimiiDe .. 3..

mantreNa nyaasaH .
aadau vedaadimuchchaarya svaraadya.n savisargakam.h .
pa~nchaarNa.n tata uddhR^itya antara.n savisargakam.h .
ante samuddharettaaraM manureshha navaaksharaH .. 4..

mudraaM bhadraarthadaatrii.n sa parashuhariNaM baahubhirbaahumekaM
    jaanvaasakta.n dadhaano bhujagabilasamaabaddhakakshyo vaTaadhaH .
aasiinashchandrakhaNDapratighaTitajaTaakshiiragaurastrinetro
    dadyaadaadyaH shukaadyairmunibhirabhivR^ito bhaavashuddhiM bhavo naH .. 5..

mantreNa nyaasaH brahmarshhinyaasaH \-
taaraM bruu.nnama uchchaarya maayaa.n vaagbhavameva cha .
dakshiNaapadamuchchaarya tataH syaanmuurtaye padam.h .. 6..

j~naana.n dehi padaM pashchaadvahnijaayaa.n tato nyaset.h .
manurashhTaadashaarNo.aya.n sarvamantreshhu gopitaH .. 7..

bhasmavyaapaaNDura~NgaH shashishakaladharo j~naanamR^idraakshamaalaa\-
     viiNaapustairviraajatkarakamaladharo yogapaTTaabhiraamaH .
vyaakhyaapiiThe nishhaNNo munivaranikaraiH sevyamaanaH prasannaH 
     savyaalaH kR^ittivaasaaH satatamavatu no dakshiNaamuurtiriishaH .. 8..

mantreNa nyaasaH . ##[##brahmarshhinyaasaH .##]##
taaraM para.n ramaabiija.n vadetsaaMbashivaaya cha .
tubhya.n chaanalajaayaa.n manurdvaadashavarNakaH .. 9..

viiNaa.n karaiH pustakamakshamaalaaM 
     bibhraaNamabhraabhagala.n varaaDhyam.h .
phaNiindrakakshyaM munibhiH shukaadyaiH 
     sevya.n vaTaadhaH kR^itaniiDamiiDe .. 10..

vishhNuu R^ishhiranushhTup chhandaH . devataa dakshiNaasyaH .
mantreNa nyaasaH . 
taara.n namo bhagavate tubhya.n vaTapada.n tataH .
muuleti padamuchchaarya vaasine padamuddharet.h .. 11..

praj~naamedhaapadaM pashchaadaadisiddhi.n tato vadet.h .
daayine padamucchaarya maayine nama uddharet.h .. 12..

vaagiishaaya tataH pashchaanmahaaj~naanapada.n tataH .
 vahnijaayaa.n tatastveshha dvaatri.nshadvarNako manuH .
aanushhTubho mantraraajaH sarvamantrottamotamaH .. 13..

dhyaanam.h .
mudraapustakavahninaagavilasadbaahuM prasannaananaM
   muktaahaaravibhuushhaNa.n shashikalaabhaasvatkiriiTojjvalam.h .
aj~naanaapahamaadimaadimagiraamarthaM bhavaaniipatiM
   nyagrodhaantanivaasinaM paraguru.n dhyaayaamyabhiishhTaaptaye .. 14..

maunamudraa .
so.ahamiti yaavadaasthitiH sanishhThaa bhavati .
tadabhedena mantraamreDana.n j~naanasaadhanam.h .
chitte tadekataanataa parikaraH . a~NgacheshhTaarpaNaM baliH .
triiNi dhaamaani kaalaH . dvaadashaantapada.n sthaanamiti .
te ha punaH shraddadhaanaastaM pratyuuchuH .
katha.n vaa.asyodayaH . ki.n svaruupam.h . ko vaa.asyopaasaka iti .
sa hovaacha . 
vairaagyatailasaMpuurNe bhaktivartisamanvite .
prabodhapuurNapaatre tu j~naptidiipa.n vilokayet.h .. 15..

mohaandhakaare niHsaare udeti svayameva hi .
vairaagyamaraNi.n kR^itvaa j~naana.n kR^itvottaraaraNim.h .. 16..

gaaDhataamisrasa.nshaantyai guuDhamartha.n nivedayet.h .
mohabhaanujasa.nkraanta.n vivekaakhyaM mR^ikaNDujam.h .. 17..

tattvaavichaarapaashena baddha.n dvaitabhayaaturam.h .
ujjiivayannijaanande svasvaruupeNa sa.nsthitaH .. 18..

shemushhii dakshiNaa proktaa saa yasyaabhiikshaNe mukham.h .
dakshiNaabhimukhaH proktaH shivo.asau brahmavaadibhiH .. 19..

sargaadikaale bhagavaanviri~nchi\-
     rupaasyaina.n sargasaamarthyamaapya .
tutoshha chitte vaa~nchhitaarthaa.nshcha labdhvaa 
     dhanyaH sopaasyopaasako bhavati dhaataa .. 20..ya imaaM paramarahasyashivatattvavidyaamadhiite sa sarvapaapebhyo mukto bhavati .
ya eva.n veda sa kaivalyamanubhavatiityupanishhat.h ..
AUM saha naavavatu . saha nau bhunaktu . saha viirya.n karavaavahai .
tejasvinaavadhiitamastu maa vidvishhaavahai ..
AUM shaantiH shaantiH shaantiH ..
iti dakshiNaamuurtyupanishhatsamaaptaa .


http://www.shaivam.org/ssudakshinamurti.htm