Or can I say, hold it down for a moment while power attacking and let go before I move too far backwards? I've found it to be completely useless so far. Any melee masters have any tips?

I usually don't perk that one as the melee range of a 1h sword is pretty poor, and when stepping back to do a power attack = miss in most cases.
Not worth it I'd say.

I know I wish that I hadn't taken it. I finally wanted to test it out on one character and I knew I'd end up regretting it. Oh well. It has to be usable in some way.

It can be used effectively with a 1H weapon, although you have to be right up in their face when you start the swing since that allows you to step back a bit and still be in range when the Power Attack goes off. If you do a forward PA followed by a backward PA you should be able to land both, since the former drives you into the opponent which sets you up to do the latter.

A 2H weapon has a bit more leeway in setting one up, thanks to the longer reach, but you still have to be close to land it and missing leaves you rather exposed.

I sometimes wish I'd skipped it myself; I tend to smith my weapons to a high degree, which renders the effect basically worthless as I'm often killing the target before it can trigger.

Sounds as tricky as the running forward power attack bonus. I have a hard time landing that one too. Either I slip past them and the swing misses or they move at the last second and it misses or I'm not doing it just right or something. About the only time it works well is when I have them crammed into a corner or up against something where they can't move. It's so hard to time it just right when I'm running toward an enemy out in the open. Too early or too late by just a bit = a miss and a lot of wasted stamina. The backwards one sounded good but I can see now it may not be worth getting.

Press and hold the power attack button, THEN press the down (S) key. Timed correctly, you don't step back at all, other than the attack animation. It also helps if you're walking instead of running.

This is one Perk that I don't think I will ever choose. I know I have not yet for any of my characters.

I would prefer it if it were like in Oblivion where you knocked the person backwards off of their feet....led to some funny physics situations. I landed the blow a lot on my H2H character, too.