If I give you a feather to throw, it is not going anywhere. But if I give you a rock to throw, you will be able to gain momentum and launch it far. This is the same concept in BJJ.
For example, if someone wants to attack me and I am on top, I want to pass that person’s guard and generate as much pressure as possible to make him uncomfortable. I want to be like a rock by putting as much pressure and becoming as heavy on the person as possible.
In the process of passing his guard, if my opponent places a foot on my hips to try and sweep me, whatever mistake I did, he may be able to lift me off the ground. At this point, it is no longer beneficial for me to be as heavy as possible – because a rock can be thrown. Instead, while in the air when he is attacking, I need to be as light as a feather in my hips, adjusting my weight accordingly. From there I will find room to put my foot back on the ground and then become a rock again.
– Professor Tony Passos during Tuesday evening’s class