Before the Match
Place your bet before the match starts. Odds are set in advance and you have more time to analyse the situation.
During the Match
Bet while the match is in progress. Odds change after every point and game, so you need to decide quickly.
Tennis is particularly suited to live betting because the pace of play shifts rapidly. A break of serve can change the outlook of an entire set. mwc's live betting interface updates odds in real time, allowing you to participate actively.
That said, it is best to avoid placing large bets in a rush during live betting. Set a budget in advance and stick to it. Follow the responsible gaming guidelines to keep betting a form of entertainment.