Burning Wheel has a subsystem called Steel. Multiple aspects of the character's build feed into this subsystem.
A PC has an initial steel rating, which is determined by reference both to some of their stats, and to various aspects of their backstory (eg having given birth to a child adds +1 to the initial rating). Steel advances in play in a fashion broadly similar to other abilities.
Steel is tested when the GM calls for a test; more on this immediately below. But for completeness, I'll also note that if the player declares a particular type of manoeuvre for their PC in a skirmish (roughly, closing with someone who is shooting at you, without taking advantage of cover or manoeuvring - ie a steady advance or charge across open ground) this is also resolved via a Steel test.
When the GM calls for a Steel test, the general rules for tests apply - ie there must be something at stake, as determined by the player-authored PC priorities (Beliefs, Instincts, traits, relationships, etc). And also, the circumstances must be the sort of circumstances that can trigger a Steel test. These are listed in the rulebook, and include fear, shock, surprise, pain, wonderment at magical effects, and attempting cold-blooded murder or similar ruthless action. The rulebook has dozens of descriptors for these sorts of circumstances, so I am not going to copy them all out. I'm trusting that what I have said conveys the gist.
The obstacle for the Steel test is Hesitation, which is rated as 10 - Will. (Assuming a mundane shade for Will - Grey (Heroic) or White (Supernatural) shade change the calculation.) There are traits which can modify hesitation, either globally or contextually (eg the cold-blooded trait reduces hesitation from murder; the fearless trait reduces hesitation from the fear of violence, but expressly excludes murder from the reduction; etc). The obstacle for the Steel test can also be modified by circumstances, eg committing murder is +2 Ob. When the test is rolled, if it it fails each degree of failure constitutes hesitation for 1 action (which is a unit of time measurement that the system uses - roughly 1 action equals 1 heartbeat, so a second or so).
The effect of this means that low Steel and low Will are likely to produce hesitation (few dice against a high Ob); whereas mediocre or better Steel and high Will have a good chance of not producing hesitation (a reasonable number of dice against a low Ob).
If the character hesitates, the player gets to choose what the character does - there is a default list, and various traits can change or add to the list. The default options are to "stand and drool" ie be too shocked or stunned to do anything; fall prone and beg for mercy; swoon; or run screaming.
There are also rules for determining how to factor actions of hesitation into various contexts of resolution, which don't use a uniform action economy (and in some cases don't use an action economy at all).