Question: what is the purpose of a jake brake
[{'answer': 'slowing the vehicle', 'provenance': [{'wikipedia_id': '886692', 'title': 'Compression release engine brake', 'start_paragraph_id': 1, 'start_character': 295, 'end_paragraph_id': 1, 'end_character': 314, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '886692', 'title': 'Compression release engine brake', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 0, 'bleu_score': 1.0, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2272980747910317181}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '886692', 'title': 'Compression release engine brake', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 314, 'bleu_score': 0.747928571980186, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2272980747910317181}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '886692', 'title': 'Compression release engine brake', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ['A compression release engine brake, frequently called a Jacobs brake or Jake brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders after the compression stroke, releasing the compressed gas trapped in the cylinders, and slowing the vehicle.', 'A compression release engine brake, frequently called a Jacobs brake or Jake brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders after the compression stroke, releasing the compressed gas trapped in the cylinders, and slowing the vehicle.', 'When activated, it opens exhaust valves in the cylinders after the compression stroke, releasing the compressed gas trapped in the cylinders, and slowing the vehicle.']}}]}]