Im fairly new to eventing and some people say when you drop into water (or grass) slip your reins... is this the right think to do? probably sounds stupid but there you go.
Yes - you may get pulled down and onto the neck which does not sit you in good stead for anything that may follow and it can hurt the horses mouth, thus promoting stopping at these fences due to the discomfort from excperiences like this. Be sure you can get the riens back fairly quiickily. If it is a small drop slip less but if it is large then more.
yes, it is the right thing to do, and it only takes a little bit of practise.
fwiw, one of Lucinda Green's exercises on her courses is to jump a small course of fences with your reins very very long (as if you've just jumped a drop, so almost on the buckle end), to see how you cope with having to keep your shoulders back and just manage somehow!
Of course in principle you should always slip your reins and lean back. This is something I have always done in quite an extreme way! Recently it was suggested not to lean back too far and to try and use my bottom more as a shock absorber, so that I didnt loose my knitting completely, particularly if you have, as is increasinly common, a skinny after a big drop.
You may also have to learn to hail a cab if your horse takes big leaps. A particularly useful emergency balance aid, where one hand naturally shoots out sideways.
Of course we should never have to do that, but it's saved me and countless others plenty !