Well, Steve - can't get the benefit of your advice in the other place, so I'll have to get it (or, rather, politely ask for it... ) here!
Fast passages up or down the length of the keyboard: I'm okay with something pretty basic like a scalic passage, or anything that keeps moving in the same direction with varying distances between the notes. But when the overall direction is upward or downward, but with deviations en route I'm less dependable. Sometimes it comes off and sometimes not. When I watch my hands I can see that it's because (at least I think it's because...) my fingers are getting ahead of my wrist, but how do I make my wrist keep up - or get ahead? It doesn't want to when it knows it has to go back in the other direction almost immediately!
Talk to your wrist gently and softly. Explain that it has a vital function to fulfil and that it will earn your undying respect and admiration if it cooperates.
You are dead right about wanting your wrist leading your fingers - your thumb has so much less distance to travel when passing under on the way up, and your 3rd\4th fingers have so much less to travel on the way down.
The change of direction doesn't have to be abrupt. Suppose you have reached your 4th\5th finger and start to descend. Your wrist can move gradually into its leading position, until it is ahead by the time you reach your thumb.
Ascending, you want your wrist in the lead by the time your thumb passes under.
It is worth while spending time consciously thinking about the movement as you practise, until it becomes second nature. To get the contrast, play a passage you have worked on in this way, but without the leading wrist - you will feel the difference immediately.
It is valuable to consider the movements involved in playing any passage you find difficult. Sometimes, a little movement of the arm here, or a rotation there can make all the difference in the world.
Did this answer your question, or have I missed the point? Hope it helps.
I think the problem is when 3 or 4 comes over on the way down; the rest of me doesn't follow quickly enough, never mind get ahead! I haven't had a chance to practise since I asked the question, though - so I'll get back to you when I have. Many thanks.
Yes, that's the piece. I don't want to bore people with it because I've been doing that for about two years now. ;D I keep putting it away in disgust, and then getting it out again, but every time it comes out again it seems more realistic. The bit I'm actually talking about that often comes off but that isn't consistent is Bar 8. I seem to be approaching it very horizontally from the right, if you know what I mean.
Yes - that other bit does sound not at all bad if I just bang out the LH and hope for the best!