Tested everything I could think of to test about rend. Here is the summary:

Rend’s base duration is 6 seconds, not 5 (the tooltip is straight up incorrect).

Rend’s selfcast damage is only calculated off mainhand. The off-hand can be any off-rolled weapon for supporting purposes (CDR, AD etc.). WW, and therefore Ambo’s, oscillates between mainhand and off-hand.

Rend scales with CHC & CHD but CAN NOT crit. Each Rend tick has (1 + CHC * CHD) as a separate multiplier. Only CHC and CHD that you see in your detail sheet, will scale rend. Any debuff-CHC (Iceblink, Multishot, Corpse Lance, Bash, etc.) will not scale Rend. Battler Rage:Bloodshed has 0 effect on/for Rend damage (except for the base rune crit), because Rend does not crit. Battle Rage:Into the Fray does not have a cap, increases CHC in sheet and therefore works with Rend.

but crit. A Rend instance snapshots CHC upon cast.

upon cast. Selfcast Rend does proc Area Damage.

Rend applied by Ambo’s does not proc Area Damage.

Ambo’s has a proc-coefficient of 1 and applies a Rend on each WWs tick.

Ambo’s does apply the equipped rune from the skillbar.

Ambo’s does oscillate between main and off-hand, since WW does.

Ambo’s pride can’t exceed the Lamentation cap of 2.

Rend:Bloodbath explosions count as a separate Rend instance but are capped at 2 due to Lamentation. This makes Bloodbath by far the best DPS rune (doubling the damage output).

Rend ticks every 12 Frames (0.2 seconds). Damage is accumulated and displayed every 48 Frames (0.8 seconds).

Untested: It’s unclear right now, if Ambo’s override selfcasted Rend instances. There are 3 outcomes:

Rend from selfcast and Rend from Ambo’s share the same cap from Lamentation and “the newest” instance overrides the “oldest” instance.

Rend from selfcast and Rend from Ambo’s share the same cap from Lamentation and “the newest” instance is ignored.

Rend from selfcast and Rend from Ambo’s have a separate cap (2 each).

It’s very tricky to conclusively test the last interaction; I can’t think of a test setup that can do it as of now, since Ambo’s resolve in 1s (a bit too quickly).

//Edit: Clarified interaction with DW and Ambo’s Pride, which wasn’t specified earlier. Since WW oscillates, Ambo’s autocast does oscillate. Rend selfcast doesn’t oscillate.