Force vertical overlap in align
I can think of two ways, either use a negative space between the first and second row, or ignore the space of the sum. I prefer the second alternative.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
P(a|b) =
\sum_{\substack{c, d, \\ e, f}} \quad\quad
&P(a,b) \\[-1.9em]
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}
\begin{align}
P(a|b) =
\smash[b]{\sum_{\substack{c, d, \\ e, f}}} \quad\quad
&P(a,b) \\
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}
\end{document}
Assuming you don't want to number each row, you can use an aligned
subblock:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
P(a|b) &=
\sum_{\substack{c, d, \\ e, f}}
\begin{aligned}[t]
\quad &P(a,b) \\
\cdot{} &P(b,c) \\
\cdot{} &P(c,d) \\
\cdot{} &P(d,e) \\
\cdot{} &P(e,f)
\end{aligned}\\
&= x+y.
\end{align}
\end{document}
However, I would prefer to offset the following lines as follows:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
P(a|b) &=
\sum_{\substack{c, d, \\ e, f}}
\begin{aligned}[t]
&P(a,b) \\
&\quad \cdot P(b,c) \\
&\quad \cdot P(c,d) \\
&\quad \cdot P(d,e) \\
&\quad \cdot P(e,f)
\end{aligned}\\
&= x+y.
\end{align}
\end{document}