The word tensaba is the imperfect form of tensar in the first person singular. There are other translations for this conjugation.
See the full tensar conjugation

tensar

En los últimos años internet tensaba los formatos propios del mundo audiovisual.
In recent years the internet has pushed the boundaries of what is possible in the world of audiovisuals.
Todo el campamento familiar se unió a la ceremonia y todos los niños se sentaron en asombrosos silencio mientras él preparaba las flechas, tensaba el arco y disparaba.
The whole of Family Camp joined in the ceremony and all the children sat in awed silence while he prepared the arrows, drew the bow and released.
Dos de los marineros tiraron de la driza mientras otro tensaba el cabo.
Two of the sailors hauled the halyard, while one more took up the slack.
La cuerda tensaba el arco, que arrastraba a la cazoleta donde se ponía la piedra cuando era liberada.
The cord tensed the bowring, which pilled the pan when it was freed.
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