HYIP or High Yield Investment Program is a Ponzi scheme. A Ponzi scheme means that the operators will offer you big returns for your investment in just a matter of days. The problem is that the money that they will give you are the new investments that other people made. So if you think about this, the operators will just be in debt if they prolong the program.
Never ever try investing in HYIP. But if you are adventurous, you could try and experience for yourself. The offers of HYIP's are so high that you are easily attracted like for example 150% in just 2 days.