A Flexi STP allows you to transfer variable amounts from one fund to the other. A minimum requisite amount for transfer is fixed but the rest depends upon the investor. You can change the amount of money you want to transfer periodically depending on your discretion and the market risk involved.
A Systematic Transfer Plan is helpful as it saves time and effort the investor would require to put in to transfer funds from one scheme to another. It helps you skip the extra steps required to reallocate funds and helps you protect at least a part of your investment.