The stock price may suddenly jump or drop after the declaration of results simply because more information is available to the analysis who study the company.
If the company is going very well, returning good profits and bagging more orders, then it would attract more investors, thereby shooting up the price. The converse also may be true.
The information that drives may become available at any time and is not necessarily tied to the time when results are declared. Since professionals track this and spend a lot of time analysing the industry and the company’s prospects they may have an upper hand compared to say, a retail investor.
Hence, invest in such stocks only if you have the time, risk appetite and skill. Else, a direct plan mutual fund might be a good alternative for you.
