TBS (The Baal Slayer) is a tab unlocked by defeating Baal for the first time. In it you can build up critical hits for your own God. Critical hits grant up to a 1000% damage boost, as well as ignoring all defense for the hit. To train tbs you start up the bar for a given location. Your goal is to click the hit button when the bar overlaps the white zone. Your progress on TBS completely resets upon rebirth, unless you buy an upgrade for God Power.
There are five locations to train:
EYES: This gives you a critical hit chance, capping at 100% crit chance. Using mirrored eyes means you must hit the button when the bar is overlapping where the mirrored image of the white zone *would be* mirrored along the middle blue line. So, if the white bar is on the left side, you want to hit where the equivalent spot would be on the right side, and vice versa. This mode gives 2.5x the reward per hit, so while it is a bit more difficult, the consensus is that it's better to use this mode vs regular.
MOUTH, WINGS, TAIL, AND FEET: These all grant critical hit damage, capping at 1000% damage overall between the four bars. You could train one zone to 900%, or split it between all 4 zones for 225% each.
There are many upgrades available for God Power, to make training TBS much easier. You can increase the width of the white zone, gain up to a 100% chance at doubling the reward for hitting, reduce the chance of a penalty to 20% when missing, and even keep up to 80% of the levels gained in TBS upon rebirth.
439 god power to gain max benefit
99 god power to max pixels at 3 pixels
100 god power to max double reward chance at 100%
160 god power to reduce chance of penalty to 20%
80 god power to max levels kept after rebirth at 80%
Baal power can be used to gain levels in this. 1 point per 1%.