Brahmasambandha is a process, where after fasting for one full day(consuming fruits and milk only) one is given the Krishna Mantra in front of a Deity "Swaroop" by a Vallabhkul Goswami, after which tulsi leaves (Indian Basil) are offered to the lotus feet of the Lord.
The Adhikaar(right) to perform daily "seva" comes only after one is initiated into Pushtimarg by means of formally granting Brahmasambandh by a Goswami Balak. Without brahmasambandh one does not hold the right to perform seva of a Pusht (alive) Swaroop (Deity.)

The Three Pillars of Pushtimarg

The three pillars of Pushtimarg are:
(1) Raag (playing and hearing traditional Haveli music)
(2) Bhog (offering pure vegetarian food that does not contain any meat or such vegetables as onion, garlic, cabbage, carrots, and a few others)
(3) Vastra & Shringar (decorating the deity with beautiful clothes and adorning the deity with jewelery)
All of the above three are included in the daily seva (devotional service) which all followers of Pushtimarg offer to their Thakurji (personal Krishna deity), and all of them have been traditionally prescribed by Goswami Shri Vitthalnathji(Shri Gusaiji) almost five hundred years ago. The raag, bhog, and vastra and shringar offerings vary daily according to the season, the date, and time of day, and this is the main reason why this path is so colourful and alive.
Seva is the most important way to attain Pushti in Pushtimarg and has been prescribed by Vallabhacharya as the fundamental tenet.