For low-energy G-mode setups, turn around after reserves finish filling but before knockback frames expire (a 4-frame window when entering G-mode with 4 reserve energy).
For high-energy (immobile) setups, instead turn around while taking knockback from the enemy that returns control to Samus (a 5-frame window)
This results in Samus entering a morph ball state without needing to have collected or equipped Morph.
See also: Tech Page