I may be a little pedantic here, but has anyone else questioned the description and function of the anvil button which increases your weight in order to make you descend faster. I hope someone else has because basic physics tells us that the weight of an object does not have any effect on the speed at which it will fall. If a heavy object and a light object were simultaneously dropped from the same height they would reach the ground at the same time. This was demonstrated on the moon using a hammer and a feather. So, sorry for being so pedantic but, I can’t help it. 
Spherical cow says moo.
Why this was not demonstrated on the earth instead is left as an exercise for the reader.
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I think your understanding of the physics involved here is a little bit out.
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in a vacuum.
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But you’re descending and not falling.
There’ll be somebody here who’s an expert in physics but my basic brain tells me the two are totally different.
Look up Galileo’s Leaning Tower of Pisa Experiment.
So, sorry guys. You can’t argue with that guy. I admit it’s a little counterintuitive, but it’s definitely true.
Whether it’s descending or falling, the only force that will be motivating the descent or fall will be gravity.
It has been demonstrated on earth multiple times.
A rolling bike on a downwards-sloping surface can’t be compared with a free-falling object.
The physics of the bike involves air resistance and rolling resistance, and mass helps overcome those. The bike doesn’t reach terminal velocity.
Which is why a two-ton bowling ball will roll down a hill a lot faster than a ping-pong ball.
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That’s only if they are different shapes, such as a feather and hammer, where air resistance would have an effect. If two spheres with the exact same size radius with one being made from lead and the other from aluminium, they would reach the ground at exactly the same time. Counter intuitive? Yes. Incorrect? no.
You are correct for free-falling objects. But this has nothing to do with objects rolling down a slope. See my reply above.
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Rolling resistance is actually increased with weight. So if you rolled down a hill on a bike, once with an added weight and once without a weight, there would be more rolling resistance with the added weight (assuming the aerodynamics hadn’t changed which it may do due to the shape of the added weight).
F (rolling resistance) = C (coefficient of rolling friction x W (weight) / r (radius of wheel)
So adding weight can never make it roll quicker. Hence the inappropriateness of the anvil. I have moved slightly towards extra weight/mass making things slower, but not quicker.
I’d agree with you 
Rolling friction is pretty insignificant (unless you’ve got a very badly maintained bik) compared to the aerodynamic resistance once you’re moving anywhere over 10kmh.
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The extra mass helps to overcome the air resistance.
Have you ever tried freewheeling down a slope with a much heavier/lighter rider alongside you? You’d see that the lighter rider has no chance if it’s a freewheeling race.
NASA disagrees with you. All else being equal, terminal velocity increases with mass.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/termvel/
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Drop two identically sized spheres at the same time, but one is a ping pong ball and the other is a lead sphere. Frontal area is the same for both so the force of air resistance holding them up is the same for both.
However the force of gravity between the Earth and the spheres is directly proportional the product of their masses. The lead sphere has more mass so the pull of Earth’s gravity on it will be greater than Earth’s pull on the ping pong ball.
This will cause the lead sphere to push through the atmosphere faster and it will hit the ground first.
The anvil power up adds 50 kg without changing the cyclist’s frontal area so the heavier cyclist descends faster.
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IRL a heavier rider descends a lot faster than a lighter rider.
It seems like the velocity of a heavier rider would be lower at first simply because there is more mass to try to accelerate, thus taking longer, but then eventually (assuming frontal area is the same) it would get faster than the lighter rider because there is greater gravitational attraction between the heavier rider and the Earth than there is between the lighter rider and the Earth.
I think.
I guess it is explained good on the NASA page linked to.
Drag increases with the square of the speed. So as an object falls, we quickly reach conditions where the drag becomes equal to the weight, if the weight is small. When drag is equal to weight, there is no net external force on the object and the vertical acceleration goes to zero. With no acceleration, the object falls at a constant velocity as described by Newton’s first law of motion. The constant vertical velocity is called the terminal velocity .
A heavier rider of the same shape is able to reach a higher terminal velocity than a lighter rider.