Conventional leftism (CL) is when you receive a multi-card clue and play the newest of them while it is not logical to do so.
For example you have Y4 Y3 in hand in this order (say second and third slots).
Y2 has just been played.
You receive a yellow clue. It's not logical to assume the leftmost one is playable, it's only conventional.
Sticking to logical leftism (LL) means you will play the newest card of a multi-card clue only if it looks like this card has been awaited for some time and you've just drawn it and your teammates give you the clue for this reason. And other logical situations.
LL may sound less efficient than CL, but it really works. Most importantly, it makes it possible to handle some situations conventional leftism can't solve (wrong timing and a multi-card clue would result in a wrong card being played) and to give pure clues that might become spoiled if you let the player draw another card.
Apart from discard order, all the advice in the following "basics" articles come with all due "in general", "most of the time", "unless..." disclaimers. They should not be followed blindly, since specific situations will sometimes encourage you to depart from them. The most important thing is that you understand the reasons behind them, in order to use them wisely.*
Remember: situation is everything and always comes first.
*"Science without conscience is but the ruin of the soul." F. Rabelais