How do you heal without opening the inventory?

So I found a tips and tricks thread guide that explains the kind of thing I was thinking of. (A little bit down in the Control Tips section).

By default, healing items are bound to numbers 7, 8, 9, 0 and are automatically sorted in your inventory from strongest to weakest at the top of your inventory. These can be rebound as well. So from your video, the player is likely hitting one of those keys that corresponds to his healing items.

Use Alphabetic numbers of your keyboard

7 = Medkit
8 = First Aid Kit
9 = Bandage
0 = Painkillers (then if no left, uses Energy Drink)

Works with QWERTY keyboards, for example AZERTY keyboards will have to remap the keys to properly use these