The word amontonas is the present form of amontonar in the second person singular.
See the full amontonar conjugation

amontonar

Conducen calor a través su longitud pero no a través — bueno, no tienen anchura, pero no en la otra dirección si los amontonas; eso es otra propiedad de la fibra de carbón también.
They conduct heat along their length but not across—well, there is no width, but not in the other direction if you stack them up; that's a property of carbon fiber also.
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