It's difficult to determine the extent of your "knee slide" with the upright support in the way. What can be said, however, is that it is highly likely that your knees are extending far past your toes - this will result in, among other things, a loss of tension in the hamstrings and the consequent failure to fully engage the posterior chain. The TUBOW cue, which has been explored and discussed in depth on this forum and in the book, would be both appropriate and useful here.