You are correct about "lash out". It is the way you use a whip. You can "lash out" verbally or physically.
Snap can have several meanings.
A snap can be a fastener on your clothes. "The coat had snaps instead of buttons".
To snap at some one, is to speak abruptly. "Her boss said, "Work faster!" "I am working faster!", she quickly snapped.
Snap can also mean break or change abruptly. "The rope will snap from the weight of the load." You can say this about a person. "After years of being kind to her angry husband, the woman finally snapped and yelled back at him."
I hope this helps :-)