Big boomhits? Hmm
I can't explain it totally in words, so this picture should help.
The thing is, high speeds help. To get to high speeds, I usually push myself off the ground with my leg. But at some point, it becomes not about high speeds, but about moderating it so you get the most penetration with your punch/kick.
pic:
You see that on the left half, I got the most penetration. That happens because, in the previous frame, I got my foot as close as possible to Uke,
but without touching him. In the next frame my foot gets really deep into Uke, and that's the most important part of boomhits for me, penetration. I got my foot to collide with a lot of joints in the picture as you see. In this case I got 4 dismemberments.
Now look at the second half of the picture. You see that my foot is pretty far away from Uke. Then I skip one frame, and my foot only penetrated a tiny bit of Uke. In this case I only got one dismemberment, and I was actually going faster. But going faster isn't necessarily better, you have to know how to moderate your speed to get your foot really close to Uke without touching him, so that in the next frame your foot gets really deep into him.
About sweet spots, the chest and crotch seem to be the most popular, but you can boom any part of Uke as long as your get 3 dismemberments or more. It is usually easier with the chest and crotch though.
inb4you already knew that