Both of the two examples given

• Bob kicked the boy, injuring his left knee.

• Congress passed the brand new tariff act, increasing the prices of imported goods.

are participles formed from relative clauses. There are other kinds of participles, like absolutes. Note that the participles in these examples can be replaced by relative clauses

• Bob kicked the boy, which injured his left knee.
  (parenthetically, this sentence is ambiguous about whose knee got injured, just like the other)

• Congress passed the brand new tariff act, which increased the prices of imported goods.

In both of these cases, the antecedent of the relative pronoun is the preceding clause.
That is,
in the first sentence, Bob kicked the boy, and that's what injured the knee, and
in the second sentence, Congress passed the tariff act, and that's what increased the prices.

This type of participle formation appears to work best with non-restrictive relative clauses, btw.

Both of the two examples given

• Bob kicked the boy, injuring his left knee.

• Congress passed the brand new tariff act, increasing the prices of imported goods.

are participles formed from relative clauses. There are other kinds of participles, like absolutes. Note that the participles in these examples can be replaced by relative clauses

• Bob kicked the boy, which injured his left knee.
  (parenthetically, this sentence is ambiguous about whose knee got injured, just like the other)

• Congress passed the brand new tariff act, which increased the prices of imported goods.

In both of these cases, the antecedent of the relative pronoun is the preceding clause.
That is,
in the first sentence, Bob kicked the boy, and that's what injured the knee, and
in the second sentence, Congress passed the tariff act, and that's what increased the prices.

This type of participle formation appears to work best with non-restrictive relative clauses, btw.