Learn Spanish (Latin America): Al tensar la caña, el sedal me cortó la mano. - Upon pulling the fishing rod taut, the fishing line cut my hand.

The structure al + infinitive is a temporal shortcut used to say "when doing something" or "upon doing something". It combines the preposition a + the masculine article el + the infinitive verb, which here is tensar (to tighten or pull taut). It is a highly common, natural way to express two actions happening simultaneously or sequentially without needing to conjugate the first verb.

Who is doing the action in al tensar? The main subject is el sedal (the fishing line), but the line didn't pull the rod taut.

Great observation! In English grammar, translating this literally as "Upon pulling the rod taut, the line cut my hand" creates a "dangling modifier" because the line isn't what pulled the rod. In Spanish, however, the al + infinitive structure is much more flexible. The implied subject of the infinitive doesn't have to match the subject of the main clause, provided the context makes it obvious. Here, the context of me (to me) makes it clear that "I" pulled the rod taut.

Doesn't caña usually mean "cane" or "reed"? How do we know it means "fishing rod" here?

You are correct that caña means cane or reed, but it is also the standard shorthand for caña de pescar (fishing rod). Because the second half of the sentence mentions el sedal (the fishing line), native speakers instantly understand from context that this refers to a fishing rod, so there is no need to add de pescar.