a system will oscillate during its transient period.i mean at that time,output will take positive and negative..if we looked at the values it will be continuously decearsing value..ie first it will attain a max positive value and then a negative value which is certainly less than positive value..further next output goes to positive which woll be less than previous positive and negative values...the first negative value is undershoot
Added after 8 minutes:
also, system may attain first value as a negative value i mean it will undershoot after that it will overshoot..correct me if i am wrong