"I've always marveled at how some people can shove those horse cock dildoes with wide flanged heads past the sigmoid bend - that's a sort of hard limit for me - pushing any kind of toy into the bend just hurts, no matter how slow and easy I go at it."
afternine,
If it hurts, don't do it. Every one says that, and they're right. They also advise a clean tunnel, lots of lube, and a slow, slow pace, not the manic thrusting of the porn videos. Also, relax, relax, relax.
Here's something you might try if you want to get into the sigmoid with a horse cock. Skip an enema, but have a recent poop. That helps with two things. One, you're fairly empty as deep as you'll probably be going. Two, the bends are now aligned for easy passage and have been recently expanded. Lube the first six inches of a fairly flexible horse cock. Lay on your side, insert just the tip, then roll back on your back and relax for five or ten minutes, just enjoying the feel of it. Next, draw your knees up to your chest and cradle your hand under the horse cock so it is lightly balanced. Then contract your anal and rectal muscles and try to draw the dildo deeper into you. If this is something you've been practicing, you'll feel the toy move a half inch or so deeper. Pause, enjoy the feeling, and then contract again.
What you'll find is that the toy all but begins to find its own way deeper. Repeat until the horse cock is bumping up against the bend into the sigmoid which is now widening itself. Then gently, ever so gently, apply a bit of push to the toy. If you're relaxed and aligned, the toy will pass through the bend with ease, with the next stop being the bend into the traverse.
This might not work the first or the second time you try it. But once it has happened for you, it's repeatable at will.