[Lipe82]

Chummers, I got a question about game mechanics in here.

Some Amps allow to reroll a certain number of d6; however, others state to reroll "failed dice". Is that just some fluff/unnecessary writing? Is there any instance in which you would choose to roll an already successful die?

ALSO, I assume that a reroll means "forget whatever result it was and apply the new one instead" correct?

[Carmody]

Of course you reroll fail dice, except for amps that allow to force opponent reroll. In that case you obviously force him to reroll his successes.
and yes, reroll means you discard the previous result and keep the new one.

[Lipe82]

Oh, that sure sounds cool.
However I don' remember any Amps that would force the target to reroll (from the official list, that is).

Can you please name one such example?

And thanks for the help, Carmody.

[Carmody]

Not so many examples, you're right, but there are a few:

Chaotic World spell p. 202

Borderline and Vector negative quality forces her to reroll hits when making social tests.