This is sort of a slow Martingale (increase when you lose) with a bit of D'Alembert (decrease when you win) strategy.
The bet selection is up to you. Originally, Harryj said you should play the losing Even Chance after it lost 3-4 times. And you bet for the next 3 spins. If a color runs for 5, 6 or 7 spins you win, if it runs for more than 7 spins you lose. The reasoning is that there are far more same color spin sequences with a length of 5, 6 or 7 spins, than there are with a length of 8 spins or more.
But you can chose a different trigger if you like.
What makes this really interesting, aside from the very balanced wagering plan, is the fact that you take a break after 3 losses. You see the trigger, you bet and if there is no win after 3 bets, you stop and wait for the next trigger. Though theoretically there is no difference, this approach protects you from long losing sequences.
Is this the best roulette strategy ever? Probably not. Is it a good starting point, a good newbie strategy? Definitely! IMO it is better than the majority of the strategies found on most sites and forums and relatively simple.