I must tell you of additional training tools I discovered while working with my students on basic walking:
The knee flexes and their body goes up and down with some students during walking practice. Now if they were to face the mirror when they walk, they could see their head rise and fall. They could put a piece of tape on the mirror at top of their head, then see if their head is going over it. My student Tom K. tells me he has a piece of string stretched out at the level of his shoulder in his "clean" attic in order to feel bobbing up and down.
In working with this for some time with my students, I discover some additional ways to train. For one, if one just touches onto a wall VERY LIGHTLY you may be able to feel the hand move up and down if the knees are flexing. We use the wall in my studio to do this and walk the entire floor lightly touching the wall. With a light enough touch, the hand provides an extra tactile sense to feel the movement of knee up and down.
Then I thought, well this is also having ancillary effect of training the hand to sense movement. Training the tactile sense of the hand in this way provides good training for "ting jin". Ting Jin as my teacher Master Stephen Hwa has said, provides you with the sensory capability in your hands to "listen" to the opponents power "surge and ebb" of movement. It has to be a very light touch however, if you bob up and down, you can feel the hand slide up and down the wall. Also, it trains the elbow not to flex, you need to hold the arm still and not do an extraneous movement where the forearm "telescopes" with the upper arm. I train myself this way with the wall at home and feeling the sensations in tips of fingers or even touching wall with back of hand or even arm, or one could even touch wall lightly with side of body. Very enjoyable developing this sense, very enjoyable.