Usually regenerative breaking is applicable to Variable Speed Drives(VSD/AFD), then it is possible to drive the motor past ts synch. speed. The VSD basically controls the V/f ration. Hope that make sense!
Thank you

Yes i understand that if you vary the the frequency and voltage in proportion lets say 400V 50Hz to 200V 25Hz the speed of the motor will decrese. I am having trouble understanding why this braking effect happens because you drive the motor past synchronous speed

The key to regen braking is keep lowering the frequency of applied voltage. Then the rotor is spinning faster than the magnetic field in the stator (This explaination is greatly simplified". When this happens the motor begins to act like a generator. The stored mechanical energy in the spinning load is converted to electrical power. A regen. drive can put this power back on the network at the appropriate frequency, or it can be "dumped" into some resistors and converted to heat.  As this process occurs the load loses speed, so the frequency of the applied voltage is again lowered and the process repeated.

The easist way (I think) to understand why this occcurs is to look at energy conservation. If the mechanical energy associated with the rotor is greater than electrical energy stored in the stator then enrgy flows from the rotor to the stator (and then to the resistors or grid). If the energy in the stator is greater than the mechanical energy in the rotor then more mechanical(or heat) energy is produced.

To brake the motor rather than say that you drive the motor past synchronous speed, it is better to think of it as lowering the synchronous speed (using the VFD) below that of the "load" which continues at speed due to momentum etc.

What happens within the machine is easier to describe in terms of a dc motor.  The current, I, can be found from the equation Vs = E + IR, where:
 
Vs is the supply voltage,
E is the back emf which helps limit the current,
R is the motor resistance.  

This is the motoring equation and current is flowing into the motor when I is +ve.
The important bit to remember is that the back emf E is proportional to speed.

If you now lower the supply voltage faster than the load can decelerate, then E will exceed Vs and I must become negative, i.e. it is reversed and flows back into the supply.  Power flow is reversed and the "load" has to supply energy to keep the machine (now a generator) turning.

With the induction motor it is similar but is text-book material involving phasor diagrams.