1)
Why did he say "Af l'Zos Yecherad Libi"?
1.
Mesilas Yesharim Perek 24: Man should tremble, for he is always in front of Hashem. It is easy to do something not proper for His awesome honor!
2.
Malbim: Thunder is a natural sound. For Hashem, it is like a weak voice. Still it makes man tremble. All the more so man would tremble if Hashem would sound the voice of a wonder above nature! If you lack strength against weak nature, how can you investigate His conduct in wonders above nature?!
2)
What is "v'Yitar"?
1.
Rashi: It jumps. This is like "Lenater Bahen Al ha'Aretz." His heart is astounded.
