Crypsis

Most blue combat tricks buy their tempo on the other side of the board: tapping a blocker, bouncing an attacker, sizing your creature past the fight. This one works entirely on your own creature, and that inversion is the design. Protection from creatures your opponents control bundles several combat-relevant effects. Before blocks, none of those creatures can declare a block against the protected body, so it walks past a wall or a swarm and connects for full. A creature-sourced tap ability that would target the protected creature is simply illegal to point at it (or is countered on resolution if it slipped through), and combat damage dealt back by an opposing blocker is prevented at resolution. The untap rider stacks a second window on top: a creature that just attacked is no longer tapped, so you can swing with one threat, hold this up, and keep a body back to defend on the crackback. The protection is deliberately partial. It reaches only creatures your opponents control, so it does nothing against burn, noncreature removal, planeswalker damage, or a fight spell whose damage comes from anything but an opposing creature. The timing has a hard edge, too: cast it after blockers are declared and the creature stays blocked, dealing its combat damage to those blockers while their damage back is prevented. That narrowness is the point: a scalpel for the creature mirror, dead weight the moment the threat to your board is anything else.