Let $X$ be a simplicial set. Let $X\to \Delta^n$ be a right fibration (has the right lifting property with respect to right horn inclusions), and let $$\Delta^{\{n-i\}}\hookrightarrow \Delta^{\{n-i,\dots, n\}}\hookrightarrow \Delta^n$$ (for a fixed $i: 0\leq i\leq n$) be the obvious inclusion maps. 


Then why is the induced map: 


$$X\times\_{\Delta^n} \Delta^{\{n-i\}} \hookrightarrow X\times\_{\Delta^n}\Delta^{\{n-i,\dots, n\}}$$ 


a deformation retract? It's not like we can apply Whitehead's theorem, since $\Delta^n$ is not a Kan complex in general.


(This is from the end of the proof of proposition 2.2.3.1 of [HTT](http://arxiv.org/pdf/math/0608040v4). The statement should be true out of the context in the book with the hypotheses I've given, but if not, there's the source.)



Why is the induced map between pullbacks (of inclusions) by a right fibration a deformation retract?
In topology, if a map $Y\to B$ is some sort of fibration then you would think that $B$ being equivalent to a subspace $A$ would imply that $Y$ is equivalent to $Y\_A=Y\times\_BA$. If fibration means Serre fibration and equivalent means weakly, then you might want to use homotopy groups and the Whitehead Theorem. But what if we try to prove directly that a deformation retraction of $B$ to $A$ would yield a deformation retraction of $Y$ to $Y\_A$? Then we will want to assume that $Y\to B$ has the 'relative homotopy lifting property' for the pair $(Y,Y\_A)$, that is, the right lifting property with respect to the inclusion $Y\times 0\cup Y\_A\times I\to Y\times I$.


I suppose that this it how it is in what you are looking at: Any right fibration of simplicial sets has the RLP w.r.t. $K\times \lbrace 1\rbrace \cup L\times \Delta^1\to K\times \Delta^1$ for every $L\subset K$ (but not w.r.t. $K\times \lbrace 0\rbrace \cup L\times \Delta^1\to K\times \Delta^1$), and this is good for lifting the sort of deformation retraction that exists from a simplex to its zeroth vertex. 


Something like that.


