i am using a 0.9V to 5V DC-DC Boost convertor but i am unable to understand its working.
Could someone please explain to me, if i am giving a 2V input how am i getting a 5V output from this module?
Where is extra voltage coming from?
The specifications of the module can be found here:

Works buy rapidly switching current from the battery through the inductor L1.
This inductor stores the energy from the current in the form of a magnetic field.
When the current flow from the battery stops, the collapsing field discharges the energy back but the current is in the opposite direction to the initial current.
This discharge produces a voltage across L1 and it appears in SERIES with the battery, so the output is the sum of the two.
Hence the increased voltage.

BUT the conservation of energy still prevails.
Power In = Power Out + Losses.

Power = Voltage x Current

If you have 5V load drawing 500mA which is 5 x .5 = 2.5Watts "Power Out"

Your input power supply will need to supply at least 2.5W ideally.
0.9 x I = 2.5Watts
I = 2.5/0.9 = 2.7Amps

I hope that is clear, anyone feel free to comment..
Just about to get a coffee.

Tom, I know you know better! The current is in the same direction, that's what inductors do: they try to maintain the same current. The voltage reverses.

I did say I was going to get a coffee to drop the blood level in my caffeine stream.
Yes, the current is in the same direction.
Could also be the reaction to first Astra shot.