The short trade
Hit, step back, and come out with more health.

« Poke, step back, then compare health bars: a trade is won on what's left, not on the fireworks. »
- His spell is down
He uses his key spell on your wave. Now it's on cooldown: he has nothing left to answer a trade.
- He steps up: you hit
He walks up to last hit a minion. That's your moment: a spell, then an attack.
- Step back right away
His minions turn on you, but you're already walking back, out of their reach. They drop you, and he can't answer.
- More health: trade won
Look at the health left: you've lost nothing, he's lost about a quarter. Whoever has more health left wins the trade.
- The classic mistake
You stay for a long trade inside his wave: his minions and his spell, back up, add up. You come out lower than him: trade lost.
When to do it
- He steps up to last hit: his eyes are on the minion, and he's closer to you.
- His key spell is on cooldown: he has nothing to answer with.
- His wave is small, or he's away from it: few minions to turn on you.
How to keep it
- A spell and an attack, no more: the trade is over before he reacts.
- Step back right away, out of his minions' reach.
- Compare the health bars after: if you have more left than him, the trade is won.
The tip for your rank
One pattern: he walks up to a minion, you hit him with your spell, you walk back. Repeat.
Watch his key spell: when he spends it on the wave, that's your window to trade until it's back.
Won trades add up: a lower opponent can't contest last hits, has to back earlier, and becomes a target for your jungler.