It’s a bonus scaling modifier on top of a base 2% based on any damage done in the last twenty seconds, not just assist damage. It caps at 10% at 800 recent damage done. Melee weapons have a base of 15%, with bonus crit modifier capped at 45%.

That used to be an absurdly strong strategy in pubs, before the crit nerf 800 damage in 30 seconds got you up to the max of ~50% crit chance, 80% for melee.

So you'd just pop around a corner as demo and airburst stickies at the heavy until you got a crit, then go ham knowing half your shots were going to be crits and it'd be trivial to keep up 800 damage until the team was wiped.