Does wearing a backpack weaken your jumping technique? I mountain bike about 4 times a week. I like to do jumps, but I wear a water backpack (water bladder, camelbak, whatever you'd like to call it). I wear it because I find that a water bottle doesn't hold enough water for me to do a long ride without having to stop to refill. My backpack holds about 100 oz of water, plus a multi tool, sunscreen, bug spray, my phone and keys, and maybe a snack or two. This means that it is heavy. I would guess that when full it is about 20 pounds. My friends tell me to take it off when doing jumps, but I find that it feels a bit awkward to take it off because I lose a significant amount of weight from doing so.
I haven't noticed any sort of difference in how easy it is to clear a given jump when my backpack is on/off, but the technique I use definitely is different when I am wearing a backpack because it adds 20 pounds hanging off my back.
Does NOT wearing a backpack actually make jumping easier, or does it just make it more comfortable? Does anyone know of a different solution to carry both a good amount of water and tools without needing a backpack?
EDIT: I forgot to say that my backpack has two straps, one that buckles at chest level and one that buckles at waist level, although no matter how tight I make it the bag still flops around due to all the tools and just stuff in general that I store in it.
You have more control over weight on you than on the bike, so long as the pack is fitted nicely and doesn't bounce around too much. This doesn't just apply to jumps but all the time, as your bodyweight is part of your control but weight on the bike is passive.  Imagine rolling over a tree root on a hardtail or even rigid forks.  Weight attached the bike must lift over the frame, but if you're off the saddle and riding properly your body and backpack float over the obstacle, barely rising.
Moving a few kg/10-20lbs between frame and body is noticeable. I'm a perpetual mountain bike novice but ride a lot on road.  On the MTB (hardtail)  I either use a hydration pack similar to you, or copy my road kit: a litre bottle of water, tool bottle in a 2nd cage, and saddlebag on the bike.  Even at my level (no big jumps, but the effect holds even for bunny-hops) bike handling is easier with the weight on my back, though it's fatiguing on a very long ride.  This makes sense.  When your weight is on your feet and hands rather than your saddle (i.e. most of the time when riding technically) your bodyweight is sprung on your legs - so is the backpack.  When you jump, you launch your body into the air and the bike follows you.  You launch a backpack with you, but extra weight on the bike has to follow you.