You do have to adjust your technique when using a foam mattress vs a traditional spring bed. I think I've mentioned it in a few posts before.
The quality (foam cell density) of the bed makes a big difference as well since "memory foam" can be a $20 mattress pad from Target that deteriorates after a year vs one's that cost over a thousand dollars. The higher end Tempur-Pedic ones have about 3' of the 'memory foam' on top of a polyester foam base. Others have alternating thickness of the types of foam for more support in specific regions. Sometimes latex is mistakenly referred to as memory foam as well.
Each density and thickness responds in a different way. The 'memory foam' is very temperature sensitive, and is much firmer and slower to conform when cold. When warm, it still takes a couple seconds to raise back up from a high pressure point like sitting up on one's knees. The stiffness of the base layer and thickness of the top layer also determines how much your knees will sink in, and how long the impression you made will take to raise back up.
Basically you need to watch out for very quick position transitions such as upright missionary on the g spot to parallel deep posterior fornix stimulation. There will be impressions from where your knees used to be, and depending on your specific bed, that means there could be steep slopes just to the side of your legs when together that you can slip down into. I was knocked out of commission for quite a while after a nasty bruise from that slip doing a practical penis stand on her pubic bone
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