The actual fit or madness the berserk experienced was known as *berserkergang*. This condition is described as follows:

This fury, which was called berserkergang, occurred not only
in the heat of battle, but also during laborious work. Men who
were thus seized performed things which otherwise seemed
impossible for human power. This condition is said to have
begun with shivering, chattering of the teeth, and chill in the
body, and then the face swelled and changed its color. With
this was connected a great hot-headedness, which at last
gave over into a great rage, under which they howled as wild
animals, bit the edge of their shields, and cut down everything
they met without dicriminating bewteen friend or foe. When
this condition ceased, a great dulling of the mind and feeble-
ness followed, which could last for one or several days
(Fabing, p. 234).

Hrolf's Saga speaks similarly of King Halfdan's berserks:

On these giants fell sometimes such a fury that they could not
control themselves, but killed men or cattle, whatever came in
their way and did not take care of itself. While this fury lasted
they were afraid of nothing, but when it left them they were so
powerless that they did not have half of their strength, and were
as feeble as if they had just come out of bed from a sickness.
This fury lasted about one day (Ibid.).

During the berserkergang, the berserk seemed to lose all human reason, a condition in which he could not distinguish between friend and enemy, and which was marked by animalistic screaming. In Arrow-Odd's Saga, Odd remarks upon hearing a group of berserkers, "Sometimes I seem to hear a bull bellowing or a dog howling, and sometimes it's like people screaming" (Edwards and Palsson, Arrow-Odd, p. 40).

This lack of awareness is clearly seen in Egils saga Skallagrimsonar, when the berserkergang came upon Egil's father, Skallagrim, as he played a ball game with his son and another young boy:

Skallagrim grew so powerful that he picked Thord up bodily and dashed him down so hard that every bone in his body was broken and he died on the spot. Then Skallagrim
grabbed Egil. Egil was saved by a servant woman, who was slain herself before Skallagrim
came out of his fit, but had she not intervened, Skallagrim would certainly
have killed his own son (Palsson and Edwards, Egil's Saga, pp. 94-95).

The berserk custom of "biting" one's shield is known from Snorri Sturlusson's Ynglinga saga, but also from the famous twelfth-century chess set found on the Isle of Lewis in the Outer Hebrides. Some of the warrior pawns in that set "bite" their shields. Biting rapidly on a shield makes a sound like that of bears clacking their teeth just before they attack. Shield-biting that sounded like threatening bears further deepened the warrior's shape-shifting trance.

The aftermath of the berserkergang was characterized by complete
physical disability. Egils saga Skallagrimssonar says:

What people say about shape-changers or those who go into
berserk fits is this: that as long as they're in the frenzy they're
so strong that nothing is too much for them, but as soon as
they're out of it they become much weaker than normal. That's
how it was with Kveldulf; as soon as the frenzy left him he felt
so worn out by the battle he'd been fighting, and grew so weak
as a result of it all that he had to take to his bed (Palsson and
Edwards, Egil's Saga, p. 72).