Biochemical Manipulation refers, specifically in Logan's case, to the manipulation of neurotransmitters within other people. This includes although isn't limited to epinephrine (a.k.a, adrenaline), norepinephrine, serotinin, and others. Given the wide range of effects this power can have, Logan is limited to a handful of specific "tricks", and some have expanded since 2011.
Chemical Negation
The most important (and money making) trick he can do comes in the form of forcibly negating an Evolved's abilities - or more accurately, suppressing them. Considering this is not like the method of negating that Trask has, Logan cannot create a field of negation and has to actively concentrate on the target in order to activate this. Season three canon tells us that adrenaline has a lot to do with the way abilities work.
Considering this is dubious science at best, this ability walks the line between what this canon tells us and what reality tells us. But for the sake of simplicity, Logan is able to negate an Evolved by manipulating their biological chemistry into switching it off for as long as his concentration lasts. As a side effect, the targeted Evolved will also feel calm (not full out sedated, just calm) and not inclined to respond to situations with the fight-or-flight instinct that is produced by norepinephrine and epinephrine.
The limits of this trick are aura-related, although he can only target one person at a time, and distance is a factor. Logan needs to be able to see someone to psychicly pick up on their biochemistry, and the maximum range is something like 30 feet. The closer he is, the quicker this ability can take into effect. At several feet away, the effect is fractionally instantaneous. At his maximum, it may take several seconds.
Panic Attack
By increasing someone's noripinephrine and adrenaline levels dramatically, Logan is able to drive someone into a panic attack. The fight-or-flight response is encouraged, in this case, driving people to want to attack or flee. In more extreme cases, they will start suffering the physical symptoms of a panic attack - shallow breathing and hyper ventilation, an increased heart rate, sweating, nausea, etc.
It can also have potentially deadly effects on people with weak hearts or are prone to asthma. It takes about ten seconds to get someone to a true panic attack. Once it's activated, it requires natural causes to shut off, whether contact is maintained or not.
Since 2011, this has become a ranged ability, with a 10 foot radius.
Good Vibrations
By manipulating the effects of serotonin in someone's nervous system, Logan has found that he has the ability to make people feel good. Panic is easier to understand than the specific functions of serotonin, and so he can't cause specific serotonin-related effects. He can't make someone who is raging mad calm down immediately, but he can be a soothing influence. He can also influence someone's sexuality and mood, which might make him seem superficially more appealing, provided their attention is on him, although the target will react naturally to whatever the circumstance is, and their own preferences of physical sexuality will influence them too.
This, like chemical negation, works on an aura-level, with a similar range of 30 feet. The effects of this ability can increase to a less subtle point if he manages skin-to-skin contact, but because this influence operates at a purely physical level, it can become obvious that he's using his ability on someone (whereas, with a panic attack, people are using too preoccupied with panicking and have less control over their personal reactions to it, unlike simply feeling more aroused for no obvious reason).
Euphoria
Logan can induce a feeling of euphoria within his target. It takes several minutes to build up. After that. the effect slowly wears away. Feelings range from giddiness, through to relaxation and general bliss. It is an obviously chemical experience - no empathic or telepathic influence whatsoever, as with his other stunts - and if someone is prone to becoming addicted to such feelings, then an addiction can manifest over time and prolonged exposure.
Since 2011, this has become a ranged ability, with a 10 foot radius.
Addiction
Active practice has allowed Logan to study the way in which biological chemicals interact to create physical dependencies to addictive substances or practices. Much like his most recently learned tricks, Logan is only able to manipulate this aspect of biological chemistry through intent, concentration and close physical contact.
By increasing the dopamine levels in a person in response to certain things, such as drug intake or his own power use, Logan is able to instill reward-seeking behaviours in the target. This takes prolonged exposure and multiple attempts to be effective, much in the same way that prolonged drug use will gradually become addictive.
On the flip side, Logan is also able to unhook someone from their poison of choice. To do this, he must "observe" someone receiving the addictive substance via his ability, and actively suppress the positive effects it has simultaneously. This does little for psychological addiction, but will take care of physiological addiction with about the same effectiveness as methadone or naltrexone treatments. Physical dependency is dampened, and behavioural desire is redirected
Biochemical Radar
Upon his power returning after spending 11 months as a technopath, Logan's gained an additional trick that does not impose his will on others, but receives a reading. He can pick up on the biochemical-related details of a person or people within a range of about 50 feet, much like he used to pick up on data as a technopath, but obviously which a much more limited range. The information he gets is not spacial, i.e., he can't tell if someone is standing one foot behind him or fifty feet in front of him behind a bush, but he can determine how many individuals are in range.
'Biochemical related details', specifically, includes roughly deducing whether someone is panicked, excited, operated under heavy medications, and so on, and also that they're even there. This power is not always on, but if Logan is in a crowd of people, it starts to activate without his control, and he has to concentrate to shut it down.
Hands Hands
The last thing that he's developed, specifically during the war and also as an after effect of his technopathic ability and his intuitive approach to it, is a sort of stabilising ability that Logan could not really describe how it works. Through skin to skin contact, Logan can kind of, in a broad way, stabilise someone on a biochemical level to an operating 'normal'. At minimum, he can bring someone does from a panic attack. At maximum, he can (for as long as he's touching them) block the effects of inebriation or symptoms relating to drug useage.