Apparently, if one travels zones as Deathtrap restores one's shield while that Deathtrap's owner has the Buck Up skill, it glitches the recipient's shield so that it's always recharging or has no recharge delay. Do I understand correctly how it happens? What is the effect exactly?
"This is the Buck Up glitch. [...] What the game views this [Buck Up] as is an active effect. Thankfully, you can store active effects in between area transfers. So, if Deathtrap is using Buck Up on you while you area-transfer, the effect of the shield regeneration gets stored on your shield and on your character, allowing a bunch of fantastic things to happen. Primarily, you'll notice on the first Buck Up stack that your shield delay for recharge rate goes to zero; your shield will recharge instantly the second that it takes any damage.
The other fantastic thing about this is that it is stackable. You can do this glitch as many times as you want during a game session, and every single time, it will increase your shield recharge rate by a greater and greater amount. There's no limit to how much you can stack this glitch or how effective it can become. [...]
So, the way that I usually set this up in a speedrun and the best way to do it is: Unequip your shield while you're near an area transfer waypoint, summon Deathtrap, go to the waypoint, and just go somewhere. And when you arrive, you'll notice that you can still see the blue outline of your shield being recharged by Deathtrap on the outside of the screen: This is gonna let you know that it works. It's very important to get the timing down properly; if you don't get the timing down properly, you're not actually activating the glitch. So, it's important that the Buck Up effect is active on you when you're transferring. You know that's happening because if you're looking from the area travel station (like this), your character will have a (you know) big blue shield ball go pop, show up in a line coming from Deathtrap to your character, to let you know that it's working. And if you're transferring in-between locations (like we did at the beginning of the video)--Let's head back to Thousand Cuts. Unequip your shield--Same--same process but the notification is a lil' different. You'll notice that, when it's active on you, again the outside of your screen is going to shine blue with the shield recharge effect, and then after you reach the next area, you will see it again."
I confirm that it still works, on version 1.8.3.
It is reset upon quitting the current game and returning to the main menu.