Ok, so this is a fairly complex question, so I'll give a somewhat basic rundown.
The bond between the Caledorian drakes and elves was originally forged by Caledor Dragontamer. I say Caledorian drakes specifically because there are differences between Elven Dragons and those that live elsewhere. Caledor went into the mountains of Caledor to commune with the dragons, and none of the elves expected him to return. When he did return, it was seated on the back of Indraugnir, who was a respected lord among the dragons even before the arrival of the Old Ones.
Caledor came up with the words of "dragontaming" which are distinct from the "dragonsong". The words of dragontaming are essentially a failsafe, for when a dragon is so wrapped up in its rage that it stops listening to its rider. The words sort of "stun" the dragon and make the bestial rage recede and allow the dragon to think clearly. That being said, it's impossible to abuse this power, since the dragon can pretty easily resist those words if it's already calm.
On the other hand, the dragonsong is how dragon riders commune with drakes. Technically, the dragonsong is really only available to elves who have formed a bond with a dragon, and it grants the elf access to the dragon's "mind", so to speak. I say "mind" because it's more complex than that, since another dragonrider in close proximity can join the bond, and both elves and both dragons can commune with each other without needing to speak. The dragonsong also makes the dragonrider susceptible to their dragon's emotions, and are often consumed by bloodlust mid-battle. This has the added effect of the dragon influencing it's rider over time, with the elf becoming more and more dragon-like in their personality.
Imrik is the current ruling prince of Caledor, and he has the greatest connection with dragons in the present-day. It's nothing near what Imladrik or Caledor Dragontamer had, but still volumes above most other dragon riders. A large part of this is because he has gained Minaithnir's (his mount) respect, which is quite rare, even among dragon-riders. Minaithnir has repaid this respect by sharing some knowledge with Imrik, including some secret dragonsongs which are more effective at waking dragons.
So basically, while the dragons are asleep in the mountains they all sorta enter this "hive-mind". Any elf trying to awaken a dragon needs to enter this hive-mind using the dragonsong in order to convince a dragon to awaken. This is actually quite a dangerous process since it consumes the elf's life-force. During an unspecified Dark Elf invasion, Imrik used his special dragonsong to try and awaken most of Ulthuan's slumbering drakes, in order to repel Malekith. He was in the caves for an unspecified amount of time, although it was at the very least a few days, potentially longer. He was continually singing the dragonsong to the point his life-force was nearly spent and was about to die, but some shenanigans happened with the Vortex and all the dragons woke up.
The difference between Imrik and Caledor Dragontamer is that Caledor Dragontamer was regarded as an equal by the greatest of the dragons, to the point where all dragons paid deference to him, calling him kalamn-kavannaen, "the great lord". Imladrik was afforded similar deference, although marginally lesser than his great-grandfather, and the dragons referred to him as kalamn-talaen, "the little lord".
Hopefully that clears up your questions.