Sub space ed head cannons?
Great ask! Let me think:
Physical pain is the one thing which puts Edward into subspace faster than anything else. Typically impact play.
Edward in true subspace is a danger to himself because he refuses to acknowledge his own limits. He will push and push and push and it's the responsibility of his partner to keep him from his own worst self.
In those moments, he could be actively bleeding and bruised with his body stressed to its limits and his most sensitive spots so overstimulated that the line between pleasure and pain is non-existent and he'll still ask for more.
He will, however, regret it all immediately in the sane aftermath as he doesn't actually enjoy the residual lingering pains as much as he thinks he will. The discomfort pisses him off actually and he can lash out and act like it's his partners fault.
He's truly beautiful in subspace. All that arrogance stripped away and replaced by a glassy-eyed, begging mess who is desperate to please and keep the noise in his brain from returning too soon.
Doesn't really experience a lot of nausea but will find himself threatened by a mild headache in the aftermath. However, he's long since learned that this can be off-set with a quick painkiller and some water.
