Hello Ankit
When we say that DTP negotiation is ON, it means that the port is in a state where, if the proper DTP packets are sent/received, the port may change its trunking functionality. Negotiation is ON in the following states: Dynamic Desirable, Dynamic Auto or Trunk.
DTP negotiation is OFF when a port is in one of the following states: Access or Non-negotiate.
As for the exchanging of DTP frames, take a look at the following NetworkLessons note that explains when DTP messages are sent.
I hope this has been helpful!
Laz