If Evan has a partial erection before a spanking, I can't resist grabbing it and giving it a squeeze. After a couple of quick squeezes, Evan's penis becomes erect. I may then ask Evan to play with it for a little while, before I stop him (no ejaculation for him: we practice tease and denial), and put him over my lap.
In this sequence of pictures, you can see that he is partially erect at first, and he becomes more erect with just a few touches.
That was only a few seconds of my touch. Now he goes over my lap, and I squeeze his erection between my thighs.
His toes are curled: I don't like this. I tell him to relax his feet, so that his toes are relaxed. He quickly complies.
With his feet the way I like them, I can now start spanking in earnest. I redden his bottom nicely in this sequence.
I will post more photos later.