How exactly does the boss rush work?

[Confused Rock]

I just beat moms heart and decided i would start looking out for it now, i looked it up, but the wiki didnt help much, unless im a complete idiot (i definiteley am a complete idiot) and im looking at it wrong. how does it work?

[Nyllia]

You have to fight waves of two bosses at a time, and if you win you get 2 extra items, there are also unlocks for each character from beating boss rush, like you unlock Undefined from beating boss rush with Eden, go to http://platinumgod.co.uk/ and you can find the unlocks

[Hypno]

When you beat Mom (the first Mom fight, not Mom's Heart/It Lives!) under 20 Minutes, a hole in the wall will open and there you find boss rush.
Please note that you need to go in there under twenty minutes of you starting the game or else it disappears (so basically if you beat Mom with a few seconds to spare and you enter the Devil Room and your time turns 20 minutes while you are there, the hole in the wall will disappear).
Inside there will be 4 Items displayed but you can only choose one.
When you do, boss rush will start.
If you manage to survive you will recieve an additional item for free.
Hope this helped :)

[Biohazard063]

Remember that it's Mom, not mom's heart.
You have a timer in-game to keep tabs on how you're doing (tap select on your Xbox controller, don't know the button when playing with keyboard) as well as the clock that shows up between floors to see if you're before or behind time-wise.

[Confused Rock]

Początkowo opublikowane przez {-NAG-}Biohazard063:

(tap select on your Xbox controller, don't know the button when playing with keyboard)

its tab i beleive. whats the clock?

[Hypno]

Początkowo opublikowane przez An confuzed rock:

Początkowo opublikowane przez {-NAG-}Biohazard063:

(tap select on your Xbox controller, don't know the button when playing with keyboard)

its tab i beleive. whats the clock?

When you go to the next floor and that short cutscene showing Isaac's dream plays you can notice a small clock on the top of the screen.
I also didn't notice it before someone actually told me.

[Biohazard063]

By small clock I also mean the image of an actual clock, not a timer.
Shows up on the line that shows you what level you're on. The place it's at means the play you "should" be already time-wise. It's basically a small reminder, you're better off keeping tabs on the actual timer. (See what I did there, now knowing it's the tab key... nvm.)

[EnafAequi]

"The Boss Rush is an optional Room that becomes available by beating Mom in less than 20 minutes"

first line on the wiki when you search for boss rush.
you can show how much time has passed by showing the big map UI (tab by default I think)