[Avodah] why stop learning?

[David Riceman]

<<R' SZN: so then the sentiment is that davening for a person , is a 
bigger zchus than learning for that person? and certainly more than 
doing a chessed for the zchus of that person? <SNIP>>>

I'm entering this in the middle of the discussion, so I don't know if 
its already been dealt with.

While I'm unsure of the mechanism by which davening for a person works, 
it has both Biblical and Mishnaic warrant.

Where is the evidence that learning or doing chessed "for the zchus" of 
a person has any effect? What mechanism induces that effect?

David Riceman