So what happened here? Either Sanji overpowered Imu here and kick his leg back, in which Imu used the momentum to increase the speed of his kick.

Or Imu used the momentum from Sanji's kick on purpose to boost his own - which is very unlikely.

Either way it was a proper clash between them and Sanji being able to react to Imu's attack and put his guard up in time is quite impressive. Unlike somebody else who got boogered...
Btw, what is this sfx sound?