Outlast is an acrylic based fibre with the ability to absorb, store and release body heat. For weft knitting there is currently only one count of yarn available - 1/30cc - this can put limitations on the knitting.
As the body heats up the wax stored in the PCMs (phase change molecules) turns to liquid (as a candle will when lit) and stores the heat. As the body cools down the wax solidifies (as a candle will when it cools down) and in so doing releases the stored heat back to the body.
Outlast products must be worn next to the skin for ultimate function.
Outlast in foam does work better due to amount of PCM that can be loaded into a foam as opposed to a fibre.
General feedback from users of Outlast is that they feel comfortable but no major changes were felt.