Domino Smash is about the other kind of dominoes — the toppling kind. You place dominoes along a course and then knock the first one over, and the chain reaction has to reach the end for the level to complete.
Gaps that are too wide break the chain and gaps that are too narrow waste dominoes, so each level is a small optimisation problem with a very satisfying payoff.
Around two-thirds of their height. Any further and the falling domino won't reach; much closer and you waste pieces without adding reliability.
Usually a gap that's slightly too wide, or a domino set at an angle so it transfers less force to the next one.
Yes — tap to place. Nothing to install.