When we train using compliance techniques (those that manipulate joints such that pain can be involved), we rely on our Ukes to slap to give some an obvious queue that they are about to feel pain.
Cool, right? we make people hurt so much that slapping themselves seems like a good idea. Not quite. Uke should slap just before pain is achieved, for one, to prevent injury, but more importantly to help Nage learn. We cannot feel each other's pain, so we must rely on communication around the threshold of pain for the attacker. Again, Uke plays a pivotal role in Nage's learning.
So why do we care about the pain threshold, just keep cranking until the cry like a baby, and wallah, success? Nope, failure. In Aikido we learn that at the threshold of pain we also have connection. Connection to Uke's One Point (mental & physical center). This connection means we own their balance and their posture and movement is ours, as Nage, to manipulate. Crossing the threshold into actual pain has its uses, but those are less important because uke will stop, crumble, or break. Connection is then lost and the dynamic nature of the exchange is ruined. This restricts Nage, making it more difficult to deal the other attackers. Oh yeah, never forget about the attacker's buddies.
The Universe is always most interesting at the thresholds, from black holes, to fractals, to Aikido.