The shields from Sonic the hedgehog in SMW, this UberASM has custom blocks and a patch alongside with it that is required for it to work, but you could also instead use the "Sonic Styled Powerdown" patch.
If you haven't played Sonic, these shields can absorb a hit (or more) for the player, and certain shields have helpful abilities.
They do not take sprite slots, only OAM slots, they do take SP1-4 GFX space though (at least one 16x16 tile)
Also, you *need* to insert this as levelASM, otherwise the shields won't display.
Tested with Asar 1.81, bsnes v115, GPS V1.4.21, Lunar Magic 3.30, SA-1 1.40, UberASM Tool 1.4.
I would suggest maybe detailing each of the base shields' abilities in the readme for those who might not be interested enough to look through or customize shieldAbilities.asm. An all-in-one customizable shield block template would also be handy for those who want a user-friendly way to set a value for the shield ID through defines.
Regardless, this is a nice implementation of this mechanic!