The rule is trivial: go long when the fast average crosses above the slow, flat or short when it crosses below. The 50 and 200 day version produces the widely reported golden-cross and death-cross.
Crossovers are honest about what they are: a lagging confirmation that the average price over a short window has moved past the average over a long one. In sustained trends they keep you in, which is the whole point of trend-following.
In ranges they are close to the worst possible tool, flipping repeatedly and producing textbook whipsaw. Most attempts to fix this add a filter, such as requiring adx above a threshold or a minimum separation between the averages, which reduces trade count rather than improving the underlying signal.
Related: golden-cross, death-cross, trend-following, whipsaw, adx