the meaning of "come in", "up to snuff" and "a piece of the back end"

[kjc0123]

What is the meaning of "come in", "up to snuff" and "a piece of the back end" in the following sentences? And how come these words have such meaning?

 

"Where, exactly, do you come in, Forrest?" 
"I am now a medical screener." 
"And what does a medical screener do?" 
"Thanks for asking. Today, for example, I was in 'a hotel suite in Dyersburg, Tennessee, helping these hefty darlings on to a treadmill. The doctor, paid by the lawyers who pay me, checks their heart capacity, and if they're not up to snuff, guess what?"
"You have a new client."
"Absolutely. Signed up forty today."
"What's the average case worth?" 
"About ten thousand bucks. The lawyers I'm now working with have eight hundred cases. That's eight million bucks, the lawyers get half, the women get screwed again. Welcome to the world of mass torts." 
"What's in it for you?" 
"A base salary, a bonus for new clients, and a piece of the back end."

[Ouisch]

"Where do you come in, Forrest?"

He's asking how Forrest fits into this particular situation, how is it applicable to him.

 

"And if they're not up to snuff..."

If they don't meet all the necessary requirements, if they don't live up to expectations.

 

"piece of the back end"

This usually refers to someone receiving a percentage of the total of what is earned.  For example, if this man is a salesman, he was to receive a regular salary, plus extra money as a bonus, and a percentage of the entire company's sales or gross profit as well.