What is $P(B_1 > 0, B_2 > 0)$ , where $B_t$ is a Brownian Motion at time $t$? The following question is found from this MSE post.
For completeness, I restate the problem below.

Question:  What is $P(B_1 > 0, B_2 > 0)$  , where $B_t$ is a Brownian Motion at time $t$?

From that post, the OP calculates the probability as follows: 

$P(B_1 > 0, B_2 > 0) = P(B_1 > 0, B_2 - B_1 > -B_1) = P(Z_1 > 0, Z_2 > -Z_1) = \frac{3}{8}$ by applying a symmetry argument to the $(Z_1, Z_2) \sim N(0, I_2)$ distribution.

I can understand all equalities except the last one which leads to the answer $\frac{3}{8}.$
In other words, I do not understand how symmetry argument to bivariate normal distribution is at play here.

First of all $Z_1$ has to be positive which happens with $\frac{1}{2}$ probability. Then we have to have $Z_2>-Z_1$ which conditioned on the previous event means that $Z_2$ is either positive (which has again $\frac{1}{2}$ probability) or it has to be negative (again $\frac{1}{2}$) and greater than $Z_1$ in absolute value - by symmetry it's once again $\frac{1}{2}$ yielding $\frac{3}{4}$ probability for that case. So the probability of the entire event is $\frac{1}{2} \cdot \frac{3}{4} = \frac{3}{8}$.
