Question

what do you mean by the line 'detention of political prisoners without trial for 2 years'

Answer 1

it means that anyone can be imprisoned for 2 years without any trial and this was an act which was passed by Britishers called Rowlatt Act in 1919.
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Answer 2

It was a provision under Rowlatt Act, 1919 that led to launch of Satyagrah by Gandhiji.Detention means the people could be arrested before Incident happensand kept in prison without any legal proceedings against them for two years
Example- In India when any tense situation becomes then some are arrested as a precautionary measure so that no unwanted incident occurs. After some time the detained people are released.