Wat is Coupling argument?

Example 11.10 (A better coin wins more often)

Draw $U_1, \dots, U_{10}$ uniform on $[0,1]$ and let toss $i$ be a success for coin $p$ if $U_i < p$. With $p = 0.6$ and $p' = 0.5$ every success of the second coin is a success of the first, so “at least seven successes” for the second implies it for the first, and $\P(\ge 7 \mid 0.6) \ge \P(\ge 7 \mid 0.5)$ without computing either (they are 0.382 and 0.172). The same construction proves that the ruin probability of Proposition 11.6 falls as $p$ rises.