In addition to previous answers:
This source (in Russian)
http://epizodsspace.airbase.ru/bibl/i_tsk/zv-reis.html
contains Gagarin's post-flight debriefing.
His exact words regarding ejection were:
"Жду катапультирования. В это время приблизительно на высоте 7 тысяч метров происходит отстрел крышки люка № 1. Хлопок, и крышка люка ушла. Я сижу и думаю, не я ли это катапультировался? Так, тихонько голову кверху повернул. В этот момент произошел выстрел, и я катапультировался. Произошло это быстро, хорошо, мягко. Ничем я не стукнулся, ничего не ушиб, все нормально. Вылетел я с креслом. Дальше стрельнула пушка, и ввелся в действие стабилизирующий парашют."
Translation:
"Waiting for ejection. In the meantime, at an altitude of approximately 7 thousand meters, shoot off (fire off) of the hatch cover No. 1 has happened. [Then I heard] the slam [or bang, meaning sharp sound], and the hatch cover was gone. I am sitting and thinking: was it me already catapulted [ejected]? [Then I] quietly (in a slow pace) turned my head up. At that moment, a shot occurred, and I ejected. It happened quickly, well [meaning satisfactorily], softly. I didn’t hit anything, didn’t bruise anything, everything was fine. I flew out with the chair. Then the gun (mortar) fired, and a stabilizing parachute (drogue chute) deployed."
I'm not 100% sure I translated in the best way his words "отстрел крышки люка" as "shoot off of the hatch cover", but I'm sure (as a native speaker) in Russian that word is tightly associated with some kind of explosive charge (whether it's gunpowder in a bullet or a TNT charge, I.e. something that rapidly blows off).
Also this web page (in Russian)
http://www.astronaut.ru/bookcase/books/spacecrash/text/08.htm
shows a sketch of the ejection (not sure, though, how authentic that is):

There are few more pictures on the web:

