Go No Sen vs Sen No Sen

Go no Sen is attacking after the first person attacks. this can be as a block
counter, a duck and counter or faking the guy out and moving out of range then
back in and countering.  
Sen no sen is striking as the other
guy is getting ready to attack, when the other guy is in the middle of attacking
or finding an opening and when the other guy starts to make a move attacking. 

So a more simpler view Go No Sen ( Go after the the attack commences) and Sen No Sen ( move before the attack commences)

Oss