Fly rythm

Thanks for the recent thread re 12 & under who can't get rythm of fly. As a 51 & under in similar boat, it was helpful, as was the Swimmer magazine article this month. So my excellent masters coach tells me I need more upper body undulation. The Nemo drill and Mary T drill (which I do in Louisville's Mary T pool BTW) are helping, but what suggestions do you have for a swim-thought while doing actual stroke? I tried today to focus on relaxing and on driving my hands forward and downward, which helped, I think. But the undulation I feel doing the stroke is much much less than what I get, for example, after I pull my arms through and leave them at my side when doing the Mary T drill. I'll also try uploading some video after the first of the year. Thanks!