yes it is simple that when btc goes up alt coin price goes down. but not sure all time will occcur the same thing with all alt coin.Some alt coin which has high value and ranked these follow the btc.when btc goes up they also go up. Maximum time when btc up alt down. so it is the best time to collect some alt coim for holding.
Usually if btc goes up, altcoins will go up as well. the price movements of altcoins tend to parallel bitcoin. I think its because the success of bitcoin also increases the success of other altcoins. If people see bitcoin doing well, they will tend to be more optimistic about the concept of cryptocurrency and buy altcoins.