Chapter IV Sloka 28

ÔVyy}aStpaey}a yaegy}aStwapre,
SvaXyay}any}aí yty> s<iztìta>

dravyayajñ˜stapoyajñ˜ 
yogayajñ˜stath˜pare |
sv˜dhy˜yajñ˜nayajñ˜þca 
yataya× saÕþitavrat˜× ( 4|28)||

Others again offer wealth, austerity and Yoga as sacrifice, while the ascetics of self-restraint and rigid vows offer study of scriptures and knowledge as sacrifice.
Commentary:

1)Some do sacrifice by distributing their wealth to the deserving as charity;
2)some offer their Tapas (austerities) as sacrifice;
3)some practise the eight limbs of Raja Yoga, viz., Yama (the five great vows), Niyama (the canons of conduct), Asana (posture), Pranayama (restraint of breath), Pratyahara (withdrawl of the senses), Dharana (concentration), Dhyana (meditation) and Samadhi (superconscious state) and offer this Yoga as a sacrifice;
4)some study the scriptures (swaadhyaaya) and offer it as sacrifice. Swaadhyaaya is self-study, meaning also introspection.
5)some do Gnana Yagna, sacrifice of knowledge, meaning renouncing of all his ignorance into the fire of knowledge kindled by him in him. This has two aspects: negation of the false and the assertion of the Real nature of the Self.

yatayah - Ascetics, persons with self-restraint.
Progress in spirituality can come only to those who are "sincere and consistent in their practices." All the five methods of Self-development can be practised only by those who are persons of "firm determination", with inexhaustible enthusiasm to reach the goal.