Fast transient loads

Typically, the term "fast transients loads" refers to some system device or low resistive load that turns on quickly and pulls a significantly amount of current. This transient event isn't long and so it is not included as part of the systems average load current/power rating but is long enough that power source's output capacitance can't accommodate, causing the power source's output voltage to dip slightly. At that point the power source's feedback loop responds to prevent the system voltage from collapsing.