If you pay close attention, when the women "steps up" or "steps onto" the man's thigh she is very carefully "placing" the blade onto a very specific area on his thigh. This move will have been practiced thousands of times (although despite all the practice and repetitions, often in competition you'll still see the woman 'hesitate' and/or be noticeably cautious during the foot placement ...often recognizable as a 1-second 'break' in music interpretation). If I recall correctly, for one of Virtue & Moir's lifts (I believe in the Olympic team event, free dance) there was a blade misalignment; and Tessa had to look back and re-place her foot before they could do the lift.
Originally Posted by rollerblade
In general, the man usually gets into a bent-knee/squat position; and then the women will "place" the blade onto a specific area on the thigh. By design, that specific area will usually be a 'balance-point' in line with the woman's 'center-of-gravity' thereby minimizing the likelihood of back & forth slicing movement.
Of course skateforever is absolutely correct that padding is used when first learning the lift/move (and even afterwards for program run-throughs until everyone is completely comfortable):
In terms of keeping the blade COMPLETELY still, you'll notice on these sorts of lifts that once up, the woman is "locked" in position. This helps ensure no movement of the blade. However, even if the woman adjusts her position slightly (which would transfer to the blade and potentially cause movement) in most cases the friction/pressure of the blade pushing down onto the thigh is enough to stop minor weight/position changes. To better explain, try this real-world example:
Originally Posted by skateforever
Take a hardcover book and place it on the top sheet of a (nicely made) bed. Push the book so it moves slightly. Now place the same hardcover book on top of your duvet or comforter. Push the book again until it moves. Notice how much more force is required to move the book while it is on the duvet/comforter. This is similar to how the blade pushes (depresses) into the thigh, which increases the frictional forces required to move it.
Or, to more closely illustrate the "inverted-U [shape of a blade] with a shallow concave in the middle", re-do the book experiment BUT place the book upright and sideways on its front & back cover edge (e.g. stand the hardcover book up so the spine is facing straight up) so that it more accurately represents the 2 concave edges of a blade.