The word tensaban is the imperfect form of tensar in the third person plural.
See the full tensar conjugation

tensar

Pasaron uno, dos, cinco túneles en donde mis nervios se tensaban al máximo con la aproximación de cada vehiculo.
I went through one; two, five tunnels were my nerves were tensed to the max with the approximation of every vehicle.
Veía como se le tensaban los músculos bajo la piel bronceada.
I could see his muscles rippling under his tanned skin.
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