What are the occasions/situations that necessitate a trust vote?
The fundamental basis on which the President appoints a prime minister is that he and his council of ministers enjoys the confidence of the majority of the House of the People (Lok Sabha). Normally it is only at the commencement of a new Parliament after general elections. However, if for some reasons the support base on which the prime minister was appointed is changed, the President can ask the prime minister to establish that he continues to enjoy a majority in the House of the People.