Also, instead of infinite retries, you have 3 lives per level and if you use them all up, you're booted back to the map.
Which you get to by moving left or right while highlighting the Options tab. For Boss Rush, you can beat the bosses in either Normal or Hero Mode and with either Normally obtained Items or just the Sword. The Boss Rush does not utilize your current save so Arzette would not start with all your Items, just what might she gets at the earliest time she can challenge the Bosses in the game. After beating each Boss, her hearts refill and she gets a Life-Up but any used items do not replenish.

For instance, you can't reach the pause menu until you are in the crouching position.

In this mode, you have one life so once you die, you can start again from the beginning. You cannot save and come back, it must be completed in one sitting. But you can keep your key items and collectibles from the levels. However, you will have no rubies or monster soul energy (unless you have the infinite soul energy powerup from the Rat Man) and the lowest collectable charges for the bombs, ropes, magic lamp oil, power stones, and canteen. You can play this in either Normal or Hero Mode and the Bosses could respawn after you restart if you so choose.

You can't save while cheats are active or earn achievements. But the cheats do have game breaking capabilities.