Rule of thumb for me: Never fully contract from being extended and vice versa, unless you're going for a nice hit or a lot of power. Remember though, there are no limits. If it looks good, do it. I hear a lot of people who say "never use your chest, just keep it held or you lose a lot of balance and it looks bad". To these people I say that they don't know what they're talking about. The chest is great for gaining momentum and increasing power. If somebody ever says not to use your chest, don't listen to them.