A Holy Lock, also known as a Spirit Lock, is a magical sealing artifact that locks down a Yokai's or hybrid being's power by sealing their blood, a process similar to the Rosario. Tsukune and Hokuto wear them, either to retain their sanity (Tsukune) or human form (Hokuto). Mikogami either created them, or is the first to have one, to surpress his Kishin nature; his torso is wrapped by three.
Unlike the rosario used by Moka Akashiya, a Holy Lock is an incomplete seal, for it only suppresses the user's power rather than fully sealing it away. This means that the user can utilize their power for short durations at varying intensities. However, this damages the seal if the release of power is done too harshly, causing a link to crack each time. This occurs because the user's power spikes at a level that the seal cannot fully lock down, damaging the seal.
This ccurred many times in the cases of Tsukune and Hokuto, where both forcefully overrode their seals to use their powers. Another aspect of this kind of seal is that it is generic in nature, being not specifically designed for a particular individual, which was the case with Tsukune. Despite this drawback, a Holy Lock is capable of adjusting to its user over time, which was revealed when Tenmei Mikougami mentioned the seal was acclimating well with Tsukune after sending Hyakushiki to attack him, as a means to help him master his then-new Vampiric powers.
Fortunately, with the right training, it is possible to access one's powers without compromising the lock by carefully drawing out a specific amount of power at any one time. This is accomplished by releasing a controlled, continuous stream of energy and gathering it in one side of the body, and allowing a small amount to flow over to the Holy Lock, where it will remain impotent. This process extends the life time of the Lock by allowing it to further develop with the user, rather than forcing it to overload in a clumsy fashion.
This occurred with Tsukune in the wake of the first battle with Fairy Tale, for Mikogami needed more combat-ready students if attacked in the future. Ruby was ordered to train Tsukune in a specially-designed dimension using the magic-cancelling whip 'Belmont' and a chain that functioned as a release valve for the seal, preventing excess energy from damaging the lock and regressing Tsukune into a Ghoul. The use of Belmont combined with the release chain enabled Tsukune to find the right method of using his power in a controlled, stable process that wouldn't damage the seal, which he was able to apply without the use of Belmont and the release chain afterwards.
Later on, Alucard reveals that the true purpose of the Holy Lock is to adapt the body to the power thats sealed off while suppressing the side effects. In other words, it's a tool used to safely turn a human into a monster.