[R] Explain how it gets back out?

[mousy0815]

Probability <- function(N, f, m, b, x, t) {
	#N is the number of lymph nodes
	#f is the fraction of Dendritic cells (in the correct node) that have the
antigen
	#m is the number of time steps
	#b is the starting position (somewhere in the node or somewhere in the gap
between nodes. It is a number between 1 and (x+t))
	#x is the number of time steps it takes to traverse the gap
	#t is the number of time steps it takes to traverse a node. 
		A <- 1/N
		B <- 1-A
		C <- 1-f
		D <- (((m+b-1)%%(x+t))+1) 

		if (b<=t) {########starts inside node
			if (m<=(t-b)){return(B + A*(C^m))} # start & end in first node		
			if (D<=t) { # we finish in a node
				a <- (B + A*(C^(t-b))) #first node
				b <- ((B + A*(C^t))^(floor((m+b)/(x+t))-1))  # intermediate nodes (if
any)
				c <- (B + A*(C^D))  # last node   
				d <- (a*b*c)	
				return(d)
				} else {Probability(N, f, (m-1), b, x, t)} ## finish in a gap	
		} else {###### starts outside node
			if (m<=(x+t-b)) {return(1)} #also end in the gap
			if (D<=t) { #end in a node
				b <- ((B + A*(C^t))^(floor((m/(x+t)))))
				c <- (B + (A*(C^D)))
				d <- (b*c)
				return(d)
				} else {Probability(N, f, (m-1), b, x, t)} #outside node
		}
	}	

I have the following code and I know it works, but I need to explain what is
going on, particularly with the recursion.
Is it true that "when each call finishes - it will pass a quantity back to
the next generation above until you return to the start of the chain, then
outputs the final result."? If so, could someone explain it in a bit more
clearly? if not, how does the recursion work - how does it finally output a
value?

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