When you are knocked down, you are temporarily invincible until you stand up. On the first frame you stand, you are vulnerable to attacks, but you can also perform any action you like, including block, throw, or an invincible move. This means your opponent has time to set up their offense for when you stand. They can time their attack so it hits you exactly as you get up, this is called a Meaty. If your opponent sets up a meaty attack on you, then if you perform any action that has startup, you will be hit. They are hitting you before your attack has a chance to even come out.
Your options against this are to block, or perform an invincible reversal move. If you block, then no harm, no foul. If you perform an invincible move exactly as you get up, such as a dragon punch or super, you can use the invincibility frames of that move to go through the opponent's attack and beat them. Depending how many invincibility frames the attack has, this may or may not work versus projectiles. Depending on the game, timing this reversal can either be really easy, or incredibly hard. And if they block your reversal, they get to punish you. Thus reversals are high risk, low reward (super reversals are high risk/high reward). Blocking on wakeup is low risk, no reward.
So lets imagine you're the attacker. What are your options versus a knocked down opponent?
- You can time a meaty, you can use your slow overhead moves without fear of them being blocked, you can go for an ambiguous crossup attack, you can throw them, or you could block their reversal. Meaty attacks on wakeup usually lead to a combo or pressure, they are low/no risk, high reward, but also a low chance of success, since opponents will block low most of the time.
- Throws have a higher chance of success, but are higher risk, especially in games with a throw invincibility timer on wakeup, because it means wakeup buttons will beat throws. SFIV and SFV both have almost no throw invincibility on wakeup, so throws will beat wakeup buttons in those games. Throws knock the opponent down again, but don't deal a lot of damage.
- Overheads can get around the opponent's low block, and are generally low/no risk in a wakeup scenario, but generally don't get as strong reward, as they cannot be canceled, are late in the cancel chain, or have initial proration for starting a combo with them.
- True Crossups are usually pretty effective on wakeup, because they avoid the front side of the character where the reversal special hits, allowing them to safely cross up and beat reversals, unless the opponent inputs their crossup backwards. Ambiguous ones are less safe, because while they have better odds of hitting the opponent, they land on top of invincible reversals more commonly. It's also difficult to hit deep enough with crossups to get a full combo in many games.
- In spite of all these options, most of them get beaten by reversals, so sometimes the right thing to do is just to block when they wake up to bait out the reversal. If you guess wrong and they do not use a reversal, you will lose your opportunity to hit your opponent, and if you're close to them, they may even wakeup throw you, but generally this is a very low risk option, however it has big rewards if the bait is successful.
- Safe Jumps are jump-in attacks timed so they'll hit meaty, but land and recover into a block before an invincible reversal can come out. They require careful timing to actually work, but have no risk if correctly performed, however they are also easily defended against, more than any other option, because their startup is reactable.
- A more advanced option would be the shimmy, basically making it look like you'll throw an opponent on their wakeup, but at the last moment stepping back so they will whiff their attempt to throw tech, or whiff a reversal, then punishing them when they miss. This is beaten by a wakeup sweep or low poke, but only works on opponents who are conditioned to perform wakeup actions.
For someone who is knocked down, your best option is generally to block low 80% of the time, and only reversal enough to prevent your opponent from getting complacent.
Wakeup scenarios are a big opportunity for an attacker to pressure their opponent with low risk options, but they can also be turned around by an unexpected reversal attack. If you're new to fighting games, be sure to practice hitting your opponent exactly when they get up. A good way to practice this in Street Fighter V is to use recording to set your opponent to do a Hard Dragon Punch move on wakeup. In SFV these are not invincible until they become active. Knock down your opponent, then practice hitting them exactly as they get up. If you do this correctly, you'll get a counter hit, and they won't Shoryuken your face.