Substitution is a technique that allows you to switch places with another object.
When you switch, “your mass” is defined to be all the mass that will need to move in order for you to move, and you need to theoretically be able to move with it. So, if you’re chained to a wall you can’t switch. If you’re carrying a sandbag, or wearing leg irons, then the sandbag / leg irons go with you. If you’re chained to a big enough lead block you can’t switch, but if the block is small enough that you could walk while dragging it then you can switch.
Momentum is also traded when switching.
Space limitation: It is not possible to move through crawl space or holes. A person must be able to walk.
Vision: You cannot do substitution without having a line-of-sight. It is possible to substitute with a target found via spyglass/telescope or other method of vision enhancement provided that you have the range for it.
Mass limitation: Target object must be within 5% of your mass. With each additional level, the difference can increase by five percent, up to a max of 99.8%. (so a 100kg person could switch with a 200g kunai) At level 20, you should have capped this out.
Time to use: fast enough that you can do it in the middle of combat. Mechanically, allows for rolling against Taijutsu/Weapons to evade an attack that's about to connect.
Time between use: There’s a cooldown after use that starts at 2s at level 1 and goes down by 0.1s per level to a minimum of 0.1s. These numbers are somewhat subjective and intended only to give a sense of things. GMs aren’t actually going to track combat at a 100ms level. At level 8 you don’t need handseals, which means you can use it while holding a weapon etc. You should capped this out at level 19(2 - 0.1x = .1 where x is the number of levels).
Disadvantages: if you're in a clear area (e.g. a hill with a training circle) or your line of sight is being interfered with (e.g. smoke bombs or MEW), and you're relying on Replacement, you're stuffed.
Advantages: when you swap, you make a Tac Mov roll using Substitution + ½ Tac Mov as your dice. This models the idea that you can move and swap together. (Note that getting out of the blast radius of an explosion is based on Tac Mov.)
Acceptable targets: It does not matter if an object is familiar or never seen before. You cannot substitute air or water or sand. But it's possible to substitute with earth and water clones.