Well, the value of the time variable increases constantly over time (boy, didn't that sound stupid ) So if you write an expression like this:

Coin.rotateY = time;

The coin would be rotated 0 degrees at 0 seconds and 1 degree at 1s and so on...

But if you write it like this:

Coin.rotateY = time * 50;

The coin would be still rotated 0 degrees at 0 secs but 50 degrees at 2 secs, 100 deg @ 3s, 150 deg @ 4s...

And the final expression Coin.rotateY = time * 50 * time; would do this:

time = 0s -> 0 * 50 * 0 -> 0 degrees.
time = 1s -> 1 * 50 * 1 -> 50 degrees.
time = 2s -> 2 * 50 * 2 -> 200 degrees.
time = 3s -> 3 * 50 * 3 -> 450 degrees.

See, the speed increases over time.