Alienation means transfer of the property. Right of disposal is one of the essential features of the ownership rights. Section 10 incorporates that any restriction on the right of disposal would be against the essential feature of ownership rights. Accordingly, section provides that if a transfer is subject to a condition by which the transferee (who now becomes the owner) is absolutely restrained from disposing of or parting with his interest in the property, the condition is void. In such cases since the transferee becomes the owner of the property, any restriction limiting his right of disposing the property would not be binding on him and he would be free to transfer it to anybody by any means.
However, there are exceptions to this rule which is unfolded in this article.
The owner of a property has a right to alienate the property either conditionally or unconditionally. Section 10 is a case of condition subsequent. In a transfer of property, where a condition is laid down by a transferor, the transfer is a "conditional transfer". Conditions are limitations which limit or otherwise affect the transfer. Condition may be condition precedent i.e., condition prior to the transfer of property and whether the transfer would take place or not is itself dependent on that condition and it may be condition subsequent where the condition is to be fulfilled after the transfer of the property. Condition subsequent affects the interest of the transferee after the transfer.
Section 10, 11, 12 and 17 of the Transfer of proeprty act provides proper calrification to your query, you may peruse the same.