at the time of starting slip S= 1, so the rotor resistance 
which depends on slip i.e. 
R2(1-S)/S           will be equzl to "0", 
i.e. rotor will act as short circuit. 
hence initailly motor will draw heavy amount of current. 
So in order to reduce this inrush current(5 to 7 times of 
rated current), we apply less input voltage at the time of 
starting so that input curent will be low(as in case of 
star connection) & after some time or specified time input 
will be fed in Delta connection.

if u understand this then ok other wise read more books in 
details...