There is a result in a paper I am reading :
Let $\Omega$ be a bounded domain. For any $\epsilon > 0$, there is a constant $C(\epsilon)$ such that $$\lVert n \times u\rVert_{H^{-1/2}(\partial \Omega)} \leq \epsilon\lVert \nabla \times u\rVert_{L^2(\Omega)} + C(\epsilon)\lVert u\rVert_{L^2(\Omega)} $$ where $n$ is the normal.
Here is the proof provided : For any $\phi \in H^{1/2}(\partial \Omega) $, consider the problem $$\nabla \times (\nabla \times w) + \frac{1}{\epsilon^2}w = 0 \text{ in } \Omega $$ $$ -n \times (n \times w) = \phi \text{ on } \partial \Omega$$ Then, the result follows immediately from estimating $|(n\times \phi)|$.
I am somewhat new to these types of arguments. Can I get a first step here? Thanks in advance.