The Siphon’s beam attack absorbs energy from nearby enemies and uses it to regenerate the health of units within its area-of-effect.
The Siphon's healing capability is most effective when a large number of allied units are within its AoE.
Targeting higher-HP enemies with the Siphon provides more efficient healing than using it to attack multiple smaller units.
While the Siphon offers powerful healing, be sure to back it up with a sizeable number of units.