It may sound like a noob-ish question but I’ve always wondered why a dry/lightly lubed bearing unresponsive and why a heavily lubed bearing responsive.
Thanks!
Because the heavy lube causes resistance keeping the bearing from spinning freely.
Sniffy-Yo.com
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Exactly,
it makes the bearing slower.
Slower=Responsive.
Yep. Like the people before me said, a slower bearing makes more resistance when spinning, and that makes it responsive.
Basically, when you tug the yo-yo, it will create a little bit of slack. That slack will start winding around the bearing, since the outer race of the bearing is spinning slowly. Soon the response system will grap the string, and force the yo-yo to come back up.
Thanks guys! 
You are correct in your answer, but your reasoning is flawed. Yes, more lube in a bearing causes it to spin less freely and because of that it will spin slower if you flick it. However, in a yo-yo the inner race of the bearing is spinning at the same rate as the yo-yo while the outer race is not spinning at all or, at most, very slowly.
Because there is more resistance inside a more heavily lubed bearing, it is isolating its two races less than a clean bearing. This, in turn, means that it requires more force to keep the outer race of a heavily lubed bearing from spinning than a clean one, or alternately, that a heavily lubed bearing is going to try harder to grab the string or climb up it than a clean bearing.
Oh, my bad. I know what I was talking about, but I guess I just didn’t type it up correctly. Thank you for correcting my mistake.
I have used thin lube. Then let it dry a bit and then use my air compressor to spray it out. Helps a lot.
mrcnja describes it pretty well. The main reason the bearing slows down is due to the viscosity of the lube. Viscosity causes drag, which slows things down. YYJ thin lube has less viscosity that say Yomega brain lube, thus it tends to slow the bearing down less.
No, no, no, you guys have it all wrong. Lube makes a bearing faster! I’ll prove it. I’m putting this marble in a metal can and and I’m going to roll it around in the can as fast as possible. Allow the marble to represent the bearing and the can the race. Now I will apply a small ammount of vegetable oil to the bottom of the can and on the bearing, to represent the lube. When I roll the marble it is going to be wicked fast. Alright I’m starting to roll the marble around and :-\ hmm… must need more oil…adding more oil and … ??? what is going on? It’s slower! That’s it I’m filling this can with oil so that marble is submerged…and now… >:( .
Like I was saying vegetable oil helps food slide down your esophagus. Here, I’ll prove it… 
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Let’s hope that marble passes through you cleanly and painlessly!