There are 3 sections here:
Rousing, Denial, and Oversexed - the features found in Toys that are used, and need to be understood first
Warming, Wetness, and Overheating - the features HoT&WeT adds
Settings - relevant settings in both HoT&WeT and Toys
1) Rousing, Denial, and Oversexed
these features are found in Toys, copied here for easy reference
Rousing
Any toy may be configured to contribute to rousing. A Rousing Strength is assigned to each arousing toy.
Over time rousing increases using the total strength of all worn toys. You will see the change in the Rousing Bar (widget), and notifications, which can both be adjusted or disabled in the MCM. If "Sync with SLA" is enabled, Rousing also changes through Sexlab Arousal, and potentially other mods that affect arousal directly using SLA. Toys is a significant contributor, but not the only contributor, and uses the term "Rousing" instead of arousal, since when not syncing, these are both separate values.
When your rousing is high enough (as set in MCM), there's a chance of a Spontaneous Orgasm, which will significantly lower your rousing, unless you successfully deny it. The same can occur in Sexlab scenes, and custom love scenes in other mods.
Rousing Benefits & Detriments
Content mods make use of rousing levels, in various and sometimes significant ways. Within Toys itself, rousing will trigger toy transformations, resulting in different effects, buffs, and escape methods. Many buffs are scaled to rousing. A buff or debuff may get better, or worse, as your rousing increases. Since rousing plays a large role in gameplay, Toys offers interactive ways to try and keep rousing up or increase it, using the following hotkeys.
Denial Hotkey
Use this hotkey (it's set in the MCM) by holding it down during a scene before squirting occurs, to attempt to deny a release. If successful you prevent a decrease in rousing and won't see or hear a squirt. If used often in a short period of time, the chance of failure increases. If you become oversexed, the chance will reach the point of nil. Fondling also contributes to oversexed.
Denial in Sexlab (if installed) Scenes behave the same as independent love scenes in Toys and Toys content mods... You squirt, each squirt reduces rousing, and the squirt can be denied using the Denial hotkey. In the MCM this can be toggled off, then Squirt/rousing reduction/denial won't happen. Useful if using a mod that alters Sexlab.
Squirting Visual for Males - The squirt visual can be turned off in the MCM. You get no visual effect, but you will hear the sound... useful for males. Rousing decrease still occurs, and you can still deny it.
Fondle Hotkey
Use this hotkey (it's defined in the MCM) to voluntarily fondle yourself. Fondle also occurs involuntarily. This will increase rousing faster than normal. If used often in a short period of time, the chance of failure increases. If you become oversexed, the chance will reach the point of nil. Denial also contributes to oversexed.
Oversexed
Oversexed increases when you successfully deny or fondle, including involuntary fondling. It decays over time. The closer you are to the maximum (which can be set in MCM), the lower the chance of fondling/denial success. You get a visual effect and slow down briefly when Oversexed is getting high.
Thermal Toys
Similar to Rousing Strength, toys can be defined to be "thermal" and are assigned a strength. Toys provides mods with total thermal strength. Toys does nothing with this value on its own. HoT&WeT is an example of a mod using Thermal.
2) Warming, Wetness, and Overheating - the features HoT&WeT adds
Warming
Thermal toys you wear warm you. The strength of their warming results from... 1) the sum of thermal values from each worn toy, and 2) your rousing level. Often a toy will need to have transformed (melted) due to high arousal to have any thermal value. You can adjust the resulting warming value in MCM.
Wetness
Squirting, which occurs during orgasm, causes you to become wet. Wetness in a cold area can be detrimental as it significantly increases your exposure. You may want to deny squirts and keep your rousing up, and avoid becoming wet. The amount of wetness from squirting can be adjusted in MCM.
Overheating
The heat from Thermal toys may eventually cause you to overheat. This happens when you are 'Completely Warm' in a warm area and wearing thermal toys over a period. Overheating decays when those conditions are not met. You can adjust the Maximum in MCM. When you overheat, you get a visual effect and are slowed briefly.
3) Settings
Gameplay that you can effect
Denial - Scenes with squinting/orgasms in Toys Content Mods... those squirts can be denied or not. Denial also increases oversexed. Most scenes have multiple squirts, thus you have the chance of full denial, partial, or none. This varies the amount of rousing loss and wetness. Includes Sexlab Scenes and non-Sexlab scenes. You are partially in control of this
Toys worn - they determine Rousing and Thermal Strength. You are partially in control of this
Fondling - You can voluntarily fondle. This increases rousing, but also oversexed. Fondling while oversexed causes minimal warmth, even if you wear no thermal toy
The key is to "manage" your rousing and oversexed. You want just enough rousing, too much gets you in trouble
Settings in HoT&WeT
Warming modifier - how thermal strength translates into warming
Wetness modifier - how wet is one squirt
Overheating max - how easily does being warm in a warm place, lead to overheating
Settings in Toys
Rousing increase/decrease modifiers - how rousing strength translates into rousing increase/decrease
Spontaneous Orgasm Chance and Level - frequency of involuntarily decreasing rousing
Fondle Chances and Level - frequency of involuntarily increasing additional rousing
Oversexed max - how easily does denial and fondling lead to failing to deny or fondle
Rousing Transformation Level - when toys transform and become warming ones