How do you heal a teammate as a Medic in KF2?

Look at the teammate and use the weapon's alt fire (middle mouse by default) to heal them. Looking at them for even half a second is enough to line it up.

Is the Hemogoblin good on non-boss waves?

Commenters disagreed. Critics said swapping to it costs enough time to lose a teammate; supporters called it a good secondary weapon whose burst heal can be stacked on one player.

Perks & Classes

KF2 How to Heal: Medic Alt Fire and Hemogoblin Debate

This is a very short, joking Steam guide titled "how to heal". It opens with a Medic scene: you are at 20 HP, and the teammate next to you holds a Hemogoblin and a Medic SMG but is not healing you. The author's "tip" is the healing basics.

The tip

Look at the teammate with 1 HP for half a second.
Use the alt fire (middle click by default).

The author then adds a second "tip" aimed at a level 25 prestiged Field Medic: stop buying the SMG and the Hemogoblin. For a deeper treatment of the perk see Field Medic perk guide and Field Medic loadout guide.

Community debate in the comments

SMG or assault rifle? One player asked why anyone should take the SMG over the assault rifle. The answer from the author was blunt: there is no good reason; the SMG costs 1500 dosh and weighs as much as two M99s.
Hemogoblin as a secondary weapon. One commenter said it works best as a secondary: use the assault rifle and shotgun most of the time and keep the Hemogoblin for its toggled burst heal, which can stack four times on one person before moving to the next. Perks also raise damage resistance, so heal the frontline first, and go left side of the skill tree for extra healing.
Hemogoblin only for boss waves? Another player asked whether it helps kill the boss faster. The reply: it deals damage over time, reduces boss damage and slows, so not really. A different commenter said it is also good for ZED Time shenanigans.
Against the Hemogoblin. One critic said the time spent equipping it on a normal wave is enough to lose a teammate.

FAQ

How do you heal a teammate as a Medic in KF2?: Look at the teammate and use the weapon's alt fire (middle mouse by default) to heal them. Looking at them for even half a second is enough to line it up.
Is the Hemogoblin good on non-boss waves?: Commenters disagreed. Critics said swapping to it costs enough time to lose a teammate; supporters called it a good secondary weapon whose burst heal can be stacked on one player.