Totally agree that this is another key for a tech lead.
I think it depends on the team and company dynamics that govern how much a team lead should 'sit and pair' with a developer.
For me personally, if there's something mission critical, and I am able to help out and stay late, I'll probably do it. Part of a tech lead is responsibly and accountability....but all within reason.
I have also been on teams with some very inexperienced devs that required way too much 'teaching and pairing' that I could do at that given moment. Mentoring/coaching is one thing, but a tech lead isn't a teacher in the literal sense...b/c their teaching is a balance given the overarching goals and progress of the team itself.
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