What is Nikon high speed sync?
Can you explain in simple words what is Nikon high speed sync;
is there any speedlight that can be used on nikon d3100 that has manual functions and also ttl functions?
High speed sync allows you to use flash at shutter speeds faster than 1/200th. Useful for fill-in but not avaialble on a D3100.
The high speed flash sync is 1/320th second but it is only available on certain cameras.
Nikon flash sync is between 1/200 to 1/250th depending upon the model number. Look in your user manual to discover what shutter speed your specific cameras sync's with flash.