It is almost like somebody doing hammer throw. Wait I will explain. Hammer is the heavy rainwater cloudes. Person carrying the hammer is the point of force - the low pressure.
On Apr 15 sun is directly above Kerala. It is hot and pressure is low. So it is pulling the rain clouds from the sea. They will reach kerala after around 1.5 months(on June 1). In fact, the clouds will always trail behind the low pressure point like a heavy hammer.
On July 21, sun is over Gujarat so is the low pressure point. Afterwards it will go back towards equator. So will the rain clouds. It will rain over north of Gujarat and then retreating monsoon will start.
On Aug 31 Kerala is again the hotest point. Within one month the rain clouds trailing behind the low pressure point will reach kerala. That is the iTavappaathi.