Can someone explain the raid boss catch technique....

1. Hold the ball without throwing it until the circle is the size you want. Depending on the boss, I'll go for either a small great or a medium excellent. To avoid accidentally "dropping" the ball, I slide my finger off the bottom of the screen (so I don't throw it forward).

2. Wait for the boss to attack. Not jump, not dodge, it has to attack.

3. Near the end of the attack animation, curve the ball and throw it as normal. You will have to throw it while the boss is still attacking. However, if you time it right, the ball will land right as the boss resets to it's resting position.

4. If the boss breaks out, you don't have to reset the circle (step 1). Just wait for the next attack and throw.

A couple notes:
- this doesn't work for lugia (or at least I can't make it work)
- you can practice on any wild Pokemon
- with this method, I almost never hit a boss while it's attacking

It doesn't work as well on Lugia because as it comes out of its attack animation the catch circle appears high up and is moving down.

I have recently reverted to the older strategy: Use a nanab berry on the first throw, then gopden razz on the remaining throws. Nanab reduces attacks by, IIRC, 80% for the first throw and 20% for all remaining throws after. That small lingering reduction helps a lot to reduce attacking twice in a row.

I've still been using this strategy on Lugia, but just tailored to the fact that the circle doesn't freeze anymore as you wind up to throw your curve ball.

I set the circle larger than I used to (to a "great" level), wait for Lugia to attack, wind up and throw, and hopefully by the time I hit it the circle has shrunk to an "excellent" level.

This doesn't work well when Lugia is floating at the higher position, so I wait until it floats down. Also, you have to keep in mind that right after Lugia does its spin attack it actually floats downward a tiny bit, so when you throw if you aim at the circle, the ball might go high. If you're going for "great" throws, this doesn't matter, you'll hit within the circle regardless. But if you're going for "excellents" though, it's best to aim a little below the circle, so you can hit it when Lugia drifts down.

The one problem I can't compensate for is if Lugia decides to do a double attack and the timing of your throw is slightly off--maybe the poster's nanab suggestion above may reduce this frequency, but I spend enough time waiting for Lugia to drift to the low position and attack as it is to try it.

There's nothing magical about it. You still have to do an accurate throw that lands in whatever size circle you set, and you have to rush your curve throw to get it off before the circle starts changing again. I was using it for a while, but it didn't work well with my throwing style. I prefer to keep the ball spinning, and wait for my moment to throw based on circle size, position of the pokemon, and my own readiness.