וְהָאֹרֵ֗ב יֹשֵׁ֥ב לָהּ֙ בַּחֶ֔דֶר וַתֹּ֣אמֶר אֵלָ֔יו פְּלִשְׁתִּ֥ים עָלֶ֖יךָ שִׁמְשׁ֑וֹן וַיְנַתֵּק֙ אֶת־הַיְתָרִ֔ים כַּאֲשֶׁ֨ר יִנָּתֵ֤ק פְּתִֽיל־הַנְּעֹ֙רֶת֙ בַּהֲרִיח֣וֹ אֵ֔שׁ וְלֹ֥א נוֹדַ֖ע כֹּחֽוֹ׃
Rashi on Judges
The source of his strength was not revealed. It was not necessary for him to exert himself when he snapped them.