Forgive my terrible phone doodle, but I think this is what's being suggested. Basically, you're groin-to-ass below, but above your chest is not pressed directly against the front partner's back. Instead, you position the lower arm so your forearm runs across their back, and wrap the upper arm over them in a half embrace.
This would work if my bed were no lt an unconscious battle field for space; if no-man’s-land exists it will be eagerly taken up, any inch given is another inch towards my partners unconscious goal of making me fall off my side of the bed. Either that or they want to be close to me. But a good thought :P
There's special pillows designed for this that have a little passthrough for your arm and a small amount of sturdy material so your arm doesn't get crushed.
Called couples pillows or or cuddle pillows or something like that. I have wanted one for some time.
Took me a while to figure out too! Turns out, it's easier for the smaller person to be a big spoon.
If you're larger than your partner, lay lower in the bed compared to your partner. Your face should be at their back. Pull that cushion down with you to help your neck. If they curl up a little, your arm can bend a bit forwards as it ends up at their legs, and it won't fall asleep at all.
My ribcage rests on a memory foam pillow. My head rests on a buckwheat pillow. My arm goes in the gap between the two pillows. The buckwheat pillow is easily remoldable so my arm can go underneath and find a good angle without making my head feel unstable.
I build a bridge out of our pillows. Two bor the base, then one spanning a small gap. I put my Earthward arm's hand up the the gap and my head on the bridge pillow.