I am not knowledgeable about technical terms for sure. I do remember another talk though on here, that someone in detail explained how java works. I think he compared it to linux, but I might be wrong. He said that not allocating all 4GB in both min and max, actually slows down the game, when it is trying to raise the ram from 2GB to 4 GB, while it is needing it. That results in lag while playing. But allocating it all at once, makes it all available to the game, so it will not have to keep trying to get more when it needs it. Also I remember I mentioned Minecraft and he (umm or she) explained that offline games do not work the same as servers, whilst Minecraft is different on that aspect. For me though, I know that adding "-Xms4g -Xmx4g" or "-Xms5g -Xmx5g" or "-Xms4g -Xmx5g", while end up with the game taking over 6GB of ram. I have tested all three, and I was seeing other apps lower their GB use, while java kept taking it all. With closed apps it would take all physical ram and still going over 6Gb and at times push to get more, lagging the system.
So I am not trying to say using it all at once is wrong, because I am sure it is not. This is why I said my machine and I did not generalize it. Even 3 GB min will end up creating a bit larger ram, always being around to 5 GB. So for me, following the instructions of the forums that suggest xms to be lower than xmx, lets me play a really heavy modded game (Nexerelin with full planets, stars and stations, SS+ and all factions that would work with Nexerelin) with some lag but no crashes, even on saving. Follow the video's suggestion, which is the right one, to set them the same, ends up with the game reaching a point that lags way too much and is unplayable, even with every other application on my machine turned off (including antivirus).