On defense leading to a new trick
Lead through strength and toward weakness
When you have no particular suits you wish to develop or ruffs you want to give your partner a good general rule is to Lead through strength or up to weakness.
What does Leading through strength mean?When it's your turn to lead to a trick you want to force the opposition player whose turn it is next to be forced to play a high card in a suit so that your partner can play an even higher card to take the trick. Lead those suits that the person playing after you has bid -- those where he has strength. If his suit is solid, you've probably lost nothing. If it's got holes, maybe your partner can get his trick right off the bat, or maybe you'll put declared to the test before he's ready.
What does Leading up to weakness mean?When it's your turn to lead to a new trick and you are on declarer's right hand side - look at dummy on the table try and lead suits that are weaker or without any winning cards. Lead those suits so your partner can win the trick using only his intermediate cards.
In this deal with North/South in 3S after you have lead the KD then AD - you have to choose, leading trumps doesn't gain your partnership any tricks but maybe the Club suit with KC JC and 7C, maybe your partner can cover whatever suit dummy plays