The ra register must be transmitted to the forked section

A sequential call sets ra to the return address.
In the example, the call to f (i.e. jal f) saves address a in ra.
A forked call (e.g. call to g) transmits a copy of the ra register before the call.
In the example, section s0 transmits address a to section s1 with the p_swcv ra,0/p_lwcv ra,0 pair.
When f ends, the ending ret instruction ends section s2 but also restarts section s0 at the return address a.