Complex numbers are couples, they have a real part and an imaginay part. Quaternions are hypercomplex numbers, they are also couples of a real part $w$ and an imaginary vector part $b = (x i, y j, z k)$.
Quaternions are tipically parameterized by two parameters: rotation angle $\theta$ and rotation axis (unit vector) $n$. The real part is $w = \cos(\frac{\theta}{2})$ while the vector part is $b = \sin(\frac{\theta}{2}) n$.
Given two qaternions $q_1 = (w_1, b_1)$ and $q_2 = (w_2, b_2)$ their multiplication in vector form is:
$$q_1 q_2 = (w_1, b_1) (w_2,b_2)$$
$$q_1 q_2 = (w_1 w_2 - b_1 \cdot b_2, w_2 b_1 + w_1 b_2 + b_1 \times b_2)$$
After multiplication the new rotation axis is:
$$b_3 = (w_2 b_1 + w_1 b_2 + b_1 \times b_2)$$
$b_3$ has one component in the plane spanned by $b_1$ and $b_2$ ($w_2 b_1 + w_1 b_2$) and one component in the direction orthogonal to that plane ($b_1 \times b_2$).
The case of the SLERP ia a little bit different. Looking at the SLERP formulae:
$$q(t) = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) q_1 + \sin(t \frac{\theta}{2}) q_2)$$
Or
$$q(t) = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) (w_1, b_1) + \sin(t \frac{\theta}{2}) (w_2, b_2))$$
Where $\theta = 2 \cos^{-1}(n_1 \cdot n_2)$ is the angle formed between the axis of rotation of $q_1$ and $q_2$. So the interpolated axis of rotation is:
$$b_t = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) b_1 + \sin(t \frac{\theta}{2}) b_2)$$
Which is in the plane spanned by $b_1$ and $b_2$ (in between $b_1$ and $b_2$ to be precise), the expected behavior of SLERP.
