A question about lower spine alignment in internal martial arts and life

A reader, Tom, commented recently on my post about training standing strengthening postures (a post I could probably add to at this point--maybe a future topic if I get around to it). Tom writes,

I'm running into some lower back issues with the flattening of the lumbar spine that occurs with the admonition to tuck (the coccyx). Lumbar discs, particularly at L5-S1, are compressed and held under compression for extended periods during YSB standing practice. It's one thing for a tuck to be a momentary phase during dynamic movement (of the spine), but to hold the tuck seems questionable to me.

The short version of my answer to this question is that I think that "tuck" is the wrong word and "drop" is the right one. For the elaborated-upon answer, keep reading.