It's the same as Rage On Hit except that it doesn't require a hit; things like Sovereign Shout for example, with +300 Rage at base. I'm not sure if increased Generation affects On Hit or not though.

Transfer Time Reduction affects how quickly one resource is converted to the other, whether it's done through spending one (and thus gaining the other) or through generating one (and thus losing the other). While in the process of Transferring, Regen isn't used in any way and doesn't matter, and you can still use Generating/On Hit skills however if you're trying to generate the same resource that's already being transferred to, the generated amount will just be added to the Transfer queue, you won't get it immediately.

To give you an example, if I spend my entire Willpower bar in an instant then the whole bar will turn black and slowly drain into Rage (takes ~14 seconds with 42% Transfer Time Reduction, ~7 seconds with 126%); while that's happening, and there's any black left between them, that's a Transfer. If I try to use a Rage generator here, it will activate as normal but it won't have any effect on either bar; my Willpower is already at 0 so there's no more to give to Rage, and it won't make it transfer any faster either. If I use the basic Staff Attack or a Rage spender though, that amount will transfer back into Willpower at the same rate per second, until the spent/generated amount is fully drained into Willpower and then it'll go back to draining the spent Willpower (but not the Willpower that I just refilled). How quickly it all happens depends ONLY on your Transfer Time Reduction stat at this stage, nothing else.

Once the black portion between the two resources is completely drained, no matter at what percentage of either bar that is, Willpower Regen takes over and will drain Rage into Willpower at a constant rate of whatever your Willpower Regen stat is per second (note: Willpower Regen and Willpower On Hit are swapped in the Show Details screen on the character page, so your Willpower Regen is whatever it says next to Willpower On Hit).

So, essentially, Generation/On Hit determine how much you transfer, while Transfer Time Reduction determines how quickly you can transfer it, while Regen sort of does both but more passively. Oh, and potions ignore all of this and give their full effect instantly.