Sanjaya uvaacha:
ityarjunam vaasudevastathoktvaa svakam roopam darshayaamaasa bhooyaha |
aashvaasayaamaasa cha bheetamenam bhootvaa punaha saumyavapoormahaatmaa || 50 ||

Sanjaya said:
Then, having said this to Arjuna, Vaasudeva showed his form, and again assuming his pleasant form, reassured the scared one.

iti : this
arjunam : to Arjuna
vaasudevaaha : Vaasudeva
tathaa : in that manner
uktvaa : saying
svakam : his
roopam : form
darshayaamaasa : showed
bhooyaha : then
aashvaasayaamaasa : reassured
cha : and
bheetam : scared one
enam : this
bhootvaa : becoming
punaha : again
saumyavapuhu : pleasant form
mahaatmaa : great one
The eighth chapter in the tenth canto (book) of the Srimad Bhaagavatam describes the ceremony where the sage Garga, in the village of Gokula, gave Shri Krishna the name “Vaasudeva” to indicate that he was the son of Vasudeva. This ceremony was conducted in a low-key manner so as not to arouse the suspicion of the king Kamsa, who had vowed to finish the progeny of Vasudeva. Vaasudeva also means “one who pervades the universe”.
The eight chapter further describes a story of Shri Krishna’s pranks. Several children approached Yashoda to complain that her son, the baby Shri Krishna, was eating dirt. Angrily, she asked Shri Krishna to open his mouth so that she can know whether he was eating dirt. When he opened his mouth, Yashoda saw a glimpse of the cosmic form inside, with all the planets, galaxies, all of time and space in that tiny mouth. She did not see the fearful version of the cosmic form shown to Arjuna. Immediately afterwards, Shri Krishna erased her memory of this incident.
In this shloka, Sanjaya introduced himself in the commentary to indicate that Shri Krishna ended the fearful cosmic form, then assumed his four armed form, and then the pleasant two armed form that Arjuna knew and loved. Shri Krishna held a whip in one hand and the reins of the chariot in another. Just like a father scolds his children and immediately pacifies them, he pacified Arjuna and ensured that his state of mind returned to normal. This is reflected in the next shloka where the chanting meter also reverts back to the “anushtubh chandha”, the default meter for chanting the Gita.