A button allowing traders to reserve some TWINS balance so as not to accidentally pay the higher trading fee
There are various possible ways to implement reserving an amount of TWINS in order to pay the cheaper rate of trading fee using TWINS.
Maybe it could be a tiny toggle button with a red light just above the right side of the [100%] button, that with the red light on the [100%] button will fill the Amount with 100% as it works now.
If you click the tiny button making it green, it could change the [100%] button to [98%], customisable in settings (to a particular amount of locked TWINS, or a chosen percentage), if desired.
A hover caption for the button could say "Save fees by reserving TWINS".