Betaine Hydrochloride (HCL) is an amino acid and is highly stimulatory to fish. It is now used to induce feeding in the fish farming industry but is also used as a human health supplement. It provides an input into the digestion of foods in humans.
You can use it neat to coat hookbaits or dilute with water and use as a dip. If diluting in water then 20g per Litre of water can be used.
Use upto 2g per kilo in boilie mixes. Using too much Betaine HCL can become a turnoff for fish like a lot of additives.